diff --git a/changelog.d/17477.feature b/changelog.d/17477.feature
new file mode 100644
index 0000000000..9785a2ef7b
--- /dev/null
+++ b/changelog.d/17477.feature
@@ -0,0 +1 @@
+Add Account Data extension support to experimental [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575) Sliding Sync `/sync` endpoint.
diff --git a/synapse/handlers/sliding_sync.py b/synapse/handlers/sliding_sync.py
index f1f6f30b95..3231574402 100644
--- a/synapse/handlers/sliding_sync.py
+++ b/synapse/handlers/sliding_sync.py
@@ -46,6 +46,7 @@ from synapse.storage.roommember import MemberSummary
 from synapse.types import (
     DeviceListUpdates,
     JsonDict,
+    JsonMapping,
     PersistedEventPosition,
     Requester,
     RoomStreamToken,
@@ -357,6 +358,7 @@ class SlidingSyncHandler:
         self.event_sources = hs.get_event_sources()
         self.relations_handler = hs.get_relations_handler()
         self.device_handler = hs.get_device_handler()
+        self.push_rules_handler = hs.get_push_rules_handler()
         self.rooms_to_exclude_globally = hs.config.server.rooms_to_exclude_from_sync
 
     async def wait_for_sync_for_user(
@@ -628,6 +630,7 @@ class SlidingSyncHandler:
 
         extensions = await self.get_extensions_response(
             sync_config=sync_config,
+            lists=lists,
             from_token=from_token,
             to_token=to_token,
         )
@@ -1797,6 +1800,7 @@ class SlidingSyncHandler:
     async def get_extensions_response(
         self,
         sync_config: SlidingSyncConfig,
+        lists: Dict[str, SlidingSyncResult.SlidingWindowList],
         to_token: StreamToken,
         from_token: Optional[SlidingSyncStreamToken],
     ) -> SlidingSyncResult.Extensions:
@@ -1804,6 +1808,7 @@ class SlidingSyncHandler:
 
         Args:
             sync_config: Sync configuration
+            lists: Sliding window API. A map of list key to list results.
             to_token: The point in the stream to sync up to.
             from_token: The point in the stream to sync from.
         """
@@ -1828,9 +1833,20 @@ class SlidingSyncHandler:
                 from_token=from_token,
             )
 
+        account_data_response = None
+        if sync_config.extensions.account_data is not None:
+            account_data_response = await self.get_account_data_extension_response(
+                sync_config=sync_config,
+                lists=lists,
+                account_data_request=sync_config.extensions.account_data,
+                to_token=to_token,
+                from_token=from_token,
+            )
+
         return SlidingSyncResult.Extensions(
             to_device=to_device_response,
             e2ee=e2ee_response,
+            account_data=account_data_response,
         )
 
     async def get_to_device_extension_response(
@@ -1956,3 +1972,125 @@ class SlidingSyncHandler:
             device_one_time_keys_count=device_one_time_keys_count,
             device_unused_fallback_key_types=device_unused_fallback_key_types,
         )
+
+    async def get_account_data_extension_response(
+        self,
+        sync_config: SlidingSyncConfig,
+        lists: Dict[str, SlidingSyncResult.SlidingWindowList],
+        account_data_request: SlidingSyncConfig.Extensions.AccountDataExtension,
+        to_token: StreamToken,
+        from_token: Optional[SlidingSyncStreamToken],
+    ) -> Optional[SlidingSyncResult.Extensions.AccountDataExtension]:
+        """Handle Account Data extension (MSC3959)
+
+        Args:
+            sync_config: Sync configuration
+            lists: Sliding window API. A map of list key to list results.
+            account_data_request: The account_data extension from the request
+            to_token: The point in the stream to sync up to.
+            from_token: The point in the stream to sync from.
+        """
+        user_id = sync_config.user.to_string()
+
+        # Skip if the extension is not enabled
+        if not account_data_request.enabled:
+            return None
+
+        global_account_data_map: Mapping[str, JsonMapping] = {}
+        if from_token is not None:
+            global_account_data_map = (
+                await self.store.get_updated_global_account_data_for_user(
+                    user_id, from_token.stream_token.account_data_key
+                )
+            )
+
+            have_push_rules_changed = await self.store.have_push_rules_changed_for_user(
+                user_id, from_token.stream_token.push_rules_key
+            )
+            if have_push_rules_changed:
+                global_account_data_map = dict(global_account_data_map)
+                global_account_data_map[AccountDataTypes.PUSH_RULES] = (
+                    await self.push_rules_handler.push_rules_for_user(sync_config.user)
+                )
+        else:
+            all_global_account_data = await self.store.get_global_account_data_for_user(
+                user_id
+            )
+
+            global_account_data_map = dict(all_global_account_data)
+            global_account_data_map[AccountDataTypes.PUSH_RULES] = (
+                await self.push_rules_handler.push_rules_for_user(sync_config.user)
+            )
+
+        # We only want to include account data for rooms that are already in the sliding
+        # sync response AND that were requested in the account data request.
+        relevant_room_ids: Set[str] = set()
+
+        # See what rooms from the room subscriptions we should get account data for
+        if (
+            account_data_request.rooms is not None
+            and sync_config.room_subscriptions is not None
+        ):
+            actual_room_ids = sync_config.room_subscriptions.keys()
+
+            for room_id in account_data_request.rooms:
+                # A wildcard means we process all rooms from the room subscriptions
+                if room_id == "*":
+                    relevant_room_ids.update(sync_config.room_subscriptions.keys())
+                    break
+
+                if room_id in actual_room_ids:
+                    relevant_room_ids.add(room_id)
+
+        # See what rooms from the sliding window lists we should get account data for
+        if account_data_request.lists is not None:
+            for list_key in account_data_request.lists:
+                # Just some typing because we share the variable name in multiple places
+                actual_list: Optional[SlidingSyncResult.SlidingWindowList] = None
+
+                # A wildcard means we process rooms from all lists
+                if list_key == "*":
+                    for actual_list in lists.values():
+                        # We only expect a single SYNC operation for any list
+                        assert len(actual_list.ops) == 1
+                        sync_op = actual_list.ops[0]
+                        assert sync_op.op == OperationType.SYNC
+
+                        relevant_room_ids.update(sync_op.room_ids)
+
+                    break
+
+                actual_list = lists.get(list_key)
+                if actual_list is not None:
+                    # We only expect a single SYNC operation for any list
+                    assert len(actual_list.ops) == 1
+                    sync_op = actual_list.ops[0]
+                    assert sync_op.op == OperationType.SYNC
+
+                    relevant_room_ids.update(sync_op.room_ids)
+
+        # Fetch room account data
+        account_data_by_room_map: Mapping[str, Mapping[str, JsonMapping]] = {}
+        if len(relevant_room_ids) > 0:
+            if from_token is not None:
+                account_data_by_room_map = (
+                    await self.store.get_updated_room_account_data_for_user(
+                        user_id, from_token.stream_token.account_data_key
+                    )
+                )
+            else:
+                account_data_by_room_map = (
+                    await self.store.get_room_account_data_for_user(user_id)
+                )
+
+        # Filter down to the relevant rooms
+        account_data_by_room_map = {
+            room_id: account_data_map
+            for room_id, account_data_map in account_data_by_room_map.items()
+            if room_id in relevant_room_ids
+        }
+
+        return SlidingSyncResult.Extensions.AccountDataExtension(
+            global_account_data_map=global_account_data_map,
+            account_data_by_room_map=account_data_by_room_map,
+        )
diff --git a/synapse/rest/client/sync.py b/synapse/rest/client/sync.py
index d72dfa2b10..7cf1f56435 100644
--- a/synapse/rest/client/sync.py
+++ b/synapse/rest/client/sync.py
@@ -929,7 +929,6 @@ class SlidingSyncRestServlet(RestServlet):
 
         return 200, response_content
 
-    # TODO: Is there a better way to encode things?
     async def encode_response(
         self,
         requester: Requester,
@@ -1117,6 +1116,24 @@ class SlidingSyncRestServlet(RestServlet):
                     extensions.e2ee.device_list_updates.left
                 )
 
+        if extensions.account_data is not None:
+            serialized_extensions["account_data"] = {
+                # Same as the the top-level `account_data.events` field in Sync v2.
+                "global": [
+                    {"type": account_data_type, "content": content}
+                    for account_data_type, content in extensions.account_data.global_account_data_map.items()
+                ],
+                # Same as the joined room's account_data field in Sync v2, e.g the path
+                # `rooms.join["!foo:bar"].account_data.events`.
+                "rooms": {
+                    room_id: [
+                        {"type": account_data_type, "content": content}
+                        for account_data_type, content in event_map.items()
+                    ]
+                    for room_id, event_map in extensions.account_data.account_data_by_room_map.items()
+                },
+            }
+
         return serialized_extensions
 
 
diff --git a/synapse/types/handlers/__init__.py b/synapse/types/handlers/__init__.py
index 59eb0963ee..479222a18d 100644
--- a/synapse/types/handlers/__init__.py
+++ b/synapse/types/handlers/__init__.py
@@ -330,11 +330,31 @@ class SlidingSyncResult:
                     or self.device_unused_fallback_key_types
                 )
 
+        @attr.s(slots=True, frozen=True, auto_attribs=True)
+        class AccountDataExtension:
+            """The Account Data extension (MSC3959)
+
+            Attributes:
+                global_account_data_map: Mapping from `type` to `content` of global account
+                    data events.
+                account_data_by_room_map: Mapping from room_id to mapping of `type` to
+                    `content` of room account data events.
+            """
+
+            global_account_data_map: Mapping[str, JsonMapping]
+            account_data_by_room_map: Mapping[str, Mapping[str, JsonMapping]]
+
+            def __bool__(self) -> bool:
+                return bool(
+                    self.global_account_data_map or self.account_data_by_room_map
+                )
+
         to_device: Optional[ToDeviceExtension] = None
         e2ee: Optional[E2eeExtension] = None
+        account_data: Optional[AccountDataExtension] = None
 
         def __bool__(self) -> bool:
-            return bool(self.to_device or self.e2ee)
+            return bool(self.to_device or self.e2ee or self.account_data)
 
     next_pos: SlidingSyncStreamToken
     lists: Dict[str, SlidingWindowList]
diff --git a/synapse/types/rest/client/__init__.py b/synapse/types/rest/client/__init__.py
index f3c45a0d6a..34e07ddac5 100644
--- a/synapse/types/rest/client/__init__.py
+++ b/synapse/types/rest/client/__init__.py
@@ -322,8 +322,26 @@ class SlidingSyncBody(RequestBodyModel):
 
             enabled: Optional[StrictBool] = False
 
+        class AccountDataExtension(RequestBodyModel):
+            """The Account Data extension (MSC3959)
+
+            Attributes:
+                enabled
+                lists: List of list keys (from the Sliding Window API) to apply this
+                    extension to.
+                rooms: List of room IDs (from the Room Subscription API) to apply this
+                    extension to.
+            """
+
+            enabled: Optional[StrictBool] = False
+            # Process all lists defined in the Sliding Window API. (This is the default.)
+            lists: Optional[List[StrictStr]] = ["*"]
+            # Process all room subscriptions defined in the Room Subscription API. (This is the default.)
+            rooms: Optional[List[StrictStr]] = ["*"]
+
         to_device: Optional[ToDeviceExtension] = None
         e2ee: Optional[E2eeExtension] = None
+        account_data: Optional[AccountDataExtension] = None
 
     # mypy workaround via https://github.com/pydantic/pydantic/issues/156#issuecomment-1130883884
     if TYPE_CHECKING:
diff --git a/tests/rest/client/test_sync.py b/tests/rest/client/test_sync.py
index 6c73f4ec33..135b677bad 100644
--- a/tests/rest/client/test_sync.py
+++ b/tests/rest/client/test_sync.py
@@ -5458,3 +5458,789 @@ class SlidingSyncE2eeExtensionTestCase(SlidingSyncBase):
             ),
             ["alg1"],
         )
+
+
+class SlidingSyncAccountDataExtensionTestCase(SlidingSyncBase):
+    """Tests for the account_data sliding sync extension"""
+
+    servlets = [
+        synapse.rest.admin.register_servlets,
+        login.register_servlets,
+        room.register_servlets,
+        sync.register_servlets,
+        sendtodevice.register_servlets,
+    ]
+
+    def default_config(self) -> JsonDict:
+        config = super().default_config()
+        # Enable sliding sync
+        config["experimental_features"] = {"msc3575_enabled": True}
+        return config
+
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        self.store = hs.get_datastores().main
+        self.event_sources = hs.get_event_sources()
+        self.e2e_keys_handler = hs.get_e2e_keys_handler()
+        self.account_data_handler = hs.get_account_data_handler()
+        self.notifier = hs.get_notifier()
+        self.sync_endpoint = (
+            "/_matrix/client/unstable/org.matrix.simplified_msc3575/sync"
+        )
+
+    def _bump_notifier_wait_for_events(self, user_id: str) -> None:
+        """
+        Wake-up a `notifier.wait_for_events(user_id)` call without affecting the Sliding
+        Sync results.
+        """
+        # We're expecting some new activity from this point onwards
+        from_token = self.event_sources.get_current_token()
+
+        triggered_notifier_wait_for_events = False
+
+        async def _on_new_acivity(
+            before_token: StreamToken, after_token: StreamToken
+        ) -> bool:
+            nonlocal triggered_notifier_wait_for_events
+            triggered_notifier_wait_for_events = True
+            return True
+
+        # Listen for some new activity for the user. We're just trying to confirm that
+        # our bump below actually does what we think it does (triggers new activity for
+        # the user).
+        result_awaitable = self.notifier.wait_for_events(
+            user_id,
+            1000,
+            _on_new_acivity,
+            from_token=from_token,
+        )
+
+        # Send a new To-Device message so that `notifier.wait_for_events(...)` wakes up.
+        # We're bumping to-device because it won't show up in the Sliding Sync response
+        # for this extension so it won't affect whether we have results.
+        sending_user_id = self.register_user(
+            "user_bump_notifier_wait_for_events", "pass"
+        )
+        sending_user_tok = self.login(sending_user_id, "pass")
+        test_msg = {"foo": "bar"}
+        chan = self.make_request(
+            "PUT",
+            "/_matrix/client/r0/sendToDevice/m.test/1234",
+            content={"messages": {user_id: {"d1": test_msg}}},
+            access_token=sending_user_tok,
+        )
+        self.assertEqual(chan.code, 200, chan.result)
+
+        # Wait for our notifier result
+        self.get_success(result_awaitable)
+
+        if not triggered_notifier_wait_for_events:
+            raise AssertionError(
+                "Expected `notifier.wait_for_events(...)` to be triggered"
+            )
+
+    def test_no_data_initial_sync(self) -> None:
+        """
+        Test that enabling the account_data extension works during an intitial sync,
+        even if there is no-data.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+
+        # Make an initial Sliding Sync request with the account_data extension enabled
+        sync_body = {
+            "lists": {},
+            "extensions": {
+                "account_data": {
+                    "enabled": True,
+                }
+            },
+        }
+        response_body, _ = self.do_sync(sync_body, tok=user1_tok)
+
+        self.assertIncludes(
+            {
+                global_event["type"]
+                for global_event in response_body["extensions"]["account_data"].get(
+                    "global"
+                )
+            },
+            # Even though we don't have any global account data set, Synapse saves some
+            # default push rules for us.
+            {AccountDataTypes.PUSH_RULES},
+            exact=True,
+        )
+        self.assertIncludes(
+            response_body["extensions"]["account_data"].get("rooms").keys(),
+            set(),
+            exact=True,
+        )
+
+    def test_no_data_incremental_sync(self) -> None:
+        """
+        Test that enabling account_data extension works during an incremental sync, even
+        if there is no-data.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+
+        sync_body = {
+            "lists": {},
+            "extensions": {
+                "account_data": {
+                    "enabled": True,
+                }
+            },
+        }
+        _, from_token = self.do_sync(sync_body, tok=user1_tok)
+
+        # Make an incremental Sliding Sync request with the account_data extension enabled
+        response_body, _ = self.do_sync(sync_body, since=from_token, tok=user1_tok)
+
+        # There has been no account data changes since the `from_token` so we shouldn't
+        # see any account data here.
+        self.assertIncludes(
+            {
+                global_event["type"]
+                for global_event in response_body["extensions"]["account_data"].get(
+                    "global"
+                )
+            },
+            set(),
+            exact=True,
+        )
+        self.assertIncludes(
+            response_body["extensions"]["account_data"].get("rooms").keys(),
+            set(),
+            exact=True,
+        )
+
+    def test_global_account_data_initial_sync(self) -> None:
+        """
+        On initial sync, we should return all global account data on initial sync.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+
+        # Update the global account data
+        self.get_success(
+            self.account_data_handler.add_account_data_for_user(
+                user_id=user1_id,
+                account_data_type="org.matrix.foobarbaz",
+                content={"foo": "bar"},
+            )
+        )
+
+        # Make an initial Sliding Sync request with the account_data extension enabled
+        sync_body = {
+            "lists": {},
+            "extensions": {
+                "account_data": {
+                    "enabled": True,
+                }
+            },
+        }
+        response_body, _ = self.do_sync(sync_body, tok=user1_tok)
+
+        # It should show us all of the global account data
+        self.assertIncludes(
+            {
+                global_event["type"]
+                for global_event in response_body["extensions"]["account_data"].get(
+                    "global"
+                )
+            },
+            {AccountDataTypes.PUSH_RULES, "org.matrix.foobarbaz"},
+            exact=True,
+        )
+        self.assertIncludes(
+            response_body["extensions"]["account_data"].get("rooms").keys(),
+            set(),
+            exact=True,
+        )
+
+    def test_global_account_data_incremental_sync(self) -> None:
+        """
+        On incremental sync, we should only account data that has changed since the
+        `from_token`.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+
+        # Add some global account data
+        self.get_success(
+            self.account_data_handler.add_account_data_for_user(
+                user_id=user1_id,
+                account_data_type="org.matrix.foobarbaz",
+                content={"foo": "bar"},
+            )
+        )
+
+        sync_body = {
+            "lists": {},
+            "extensions": {
+                "account_data": {
+                    "enabled": True,
+                }
+            },
+        }
+        _, from_token = self.do_sync(sync_body, tok=user1_tok)
+
+        # Add some other global account data
+        self.get_success(
+            self.account_data_handler.add_account_data_for_user(
+                user_id=user1_id,
+                account_data_type="org.matrix.doodardaz",
+                content={"doo": "dar"},
+            )
+        )
+
+        # Make an incremental Sliding Sync request with the account_data extension enabled
+        response_body, _ = self.do_sync(sync_body, since=from_token, tok=user1_tok)
+
+        self.assertIncludes(
+            {
+                global_event["type"]
+                for global_event in response_body["extensions"]["account_data"].get(
+                    "global"
+                )
+            },
+            # We should only see the new global account data that happened after the `from_token`
+            {"org.matrix.doodardaz"},
+            exact=True,
+        )
+        self.assertIncludes(
+            response_body["extensions"]["account_data"].get("rooms").keys(),
+            set(),
+            exact=True,
+        )
+
+    def test_room_account_data_initial_sync(self) -> None:
+        """
+        On initial sync, we return all account data for a given room but only for
+        rooms that we request and are being returned in the Sliding Sync response.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+
+        # Create a room and add some room account data
+        room_id1 = self.helper.create_room_as(user1_id, tok=user1_tok)
+        self.get_success(
+            self.account_data_handler.add_account_data_to_room(
+                user_id=user1_id,
+                room_id=room_id1,
+                account_data_type="org.matrix.roorarraz",
+                content={"roo": "rar"},
+            )
+        )
+
+        # Create another room with some room account data
+        room_id2 = self.helper.create_room_as(user1_id, tok=user1_tok)
+        self.get_success(
+            self.account_data_handler.add_account_data_to_room(
+                user_id=user1_id,
+                room_id=room_id2,
+                account_data_type="org.matrix.roorarraz",
+                content={"roo": "rar"},
+            )
+        )
+
+        # Make an initial Sliding Sync request with the account_data extension enabled
+        sync_body = {
+            "lists": {},
+            "room_subscriptions": {
+                room_id1: {
+                    "required_state": [],
+                    "timeline_limit": 0,
+                }
+            },
+            "extensions": {
+                "account_data": {
+                    "enabled": True,
+                    "rooms": [room_id1, room_id2],
+                }
+            },
+        }
+        response_body, _ = self.do_sync(sync_body, tok=user1_tok)
+
+        self.assertIsNotNone(response_body["extensions"]["account_data"].get("global"))
+        # Even though we requested room2, we only expect room1 to show up because that's
+        # the only room in the Sliding Sync response (room2 is not one of our room
+        # subscriptions or in a sliding window list).
+        self.assertIncludes(
+            response_body["extensions"]["account_data"].get("rooms").keys(),
+            {room_id1},
+            exact=True,
+        )
+        self.assertIncludes(
+            {
+                event["type"]
+                for event in response_body["extensions"]["account_data"]
+                .get("rooms")
+                .get(room_id1)
+            },
+            {"org.matrix.roorarraz"},
+            exact=True,
+        )
+
+    def test_room_account_data_incremental_sync(self) -> None:
+        """
+        On incremental sync, we return all account data for a given room but only for
+        rooms that we request and are being returned in the Sliding Sync response.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+
+        # Create a room and add some room account data
+        room_id1 = self.helper.create_room_as(user1_id, tok=user1_tok)
+        self.get_success(
+            self.account_data_handler.add_account_data_to_room(
+                user_id=user1_id,
+                room_id=room_id1,
+                account_data_type="org.matrix.roorarraz",
+                content={"roo": "rar"},
+            )
+        )
+
+        # Create another room with some room account data
+        room_id2 = self.helper.create_room_as(user1_id, tok=user1_tok)
+        self.get_success(
+            self.account_data_handler.add_account_data_to_room(
+                user_id=user1_id,
+                room_id=room_id2,
+                account_data_type="org.matrix.roorarraz",
+                content={"roo": "rar"},
+            )
+        )
+
+        sync_body = {
+            "lists": {},
+            "room_subscriptions": {
+                room_id1: {
+                    "required_state": [],
+                    "timeline_limit": 0,
+                }
+            },
+            "extensions": {
+                "account_data": {
+                    "enabled": True,
+                    "rooms": [room_id1, room_id2],
+                }
+            },
+        }
+        _, from_token = self.do_sync(sync_body, tok=user1_tok)
+
+        # Add some other room account data
+        self.get_success(
+            self.account_data_handler.add_account_data_to_room(
+                user_id=user1_id,
+                room_id=room_id1,
+                account_data_type="org.matrix.roorarraz2",
+                content={"roo": "rar"},
+            )
+        )
+        self.get_success(
+            self.account_data_handler.add_account_data_to_room(
+                user_id=user1_id,
+                room_id=room_id2,
+                account_data_type="org.matrix.roorarraz2",
+                content={"roo": "rar"},
+            )
+        )
+
+        # Make an incremental Sliding Sync request with the account_data extension enabled
+        response_body, _ = self.do_sync(sync_body, since=from_token, tok=user1_tok)
+
+        self.assertIsNotNone(response_body["extensions"]["account_data"].get("global"))
+        # Even though we requested room2, we only expect room1 to show up because that's
+        # the only room in the Sliding Sync response (room2 is not one of our room
+        # subscriptions or in a sliding window list).
+        self.assertIncludes(
+            response_body["extensions"]["account_data"].get("rooms").keys(),
+            {room_id1},
+            exact=True,
+        )
+        # We should only see the new room account data that happened after the `from_token`
+        self.assertIncludes(
+            {
+                event["type"]
+                for event in response_body["extensions"]["account_data"]
+                .get("rooms")
+                .get(room_id1)
+            },
+            {"org.matrix.roorarraz2"},
+            exact=True,
+        )
+
+    def test_room_account_data_relevant_rooms(self) -> None:
+        """
+        Test out different variations of `lists`/`rooms` we are requesting account data for.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+
+        # Create a room and add some room account data
+        room_id1 = self.helper.create_room_as(user1_id, tok=user1_tok)
+        self.get_success(
+            self.account_data_handler.add_account_data_to_room(
+                user_id=user1_id,
+                room_id=room_id1,
+                account_data_type="org.matrix.roorarraz",
+                content={"roo": "rar"},
+            )
+        )
+
+        # Create another room with some room account data
+        room_id2 = self.helper.create_room_as(user1_id, tok=user1_tok)
+        self.get_success(
+            self.account_data_handler.add_account_data_to_room(
+                user_id=user1_id,
+                room_id=room_id2,
+                account_data_type="org.matrix.roorarraz",
+                content={"roo": "rar"},
+            )
+        )
+
+        # Create another room with some room account data
+        room_id3 = self.helper.create_room_as(user1_id, tok=user1_tok)
+        self.get_success(
+            self.account_data_handler.add_account_data_to_room(
+                user_id=user1_id,
+                room_id=room_id3,
+                account_data_type="org.matrix.roorarraz",
+                content={"roo": "rar"},
+            )
+        )
+
+        # Create another room with some room account data
+        room_id4 = self.helper.create_room_as(user1_id, tok=user1_tok)
+        self.get_success(
+            self.account_data_handler.add_account_data_to_room(
+                user_id=user1_id,
+                room_id=room_id4,
+                account_data_type="org.matrix.roorarraz",
+                content={"roo": "rar"},
+            )
+        )
+
+        # Create another room with some room account data
+        room_id5 = self.helper.create_room_as(user1_id, tok=user1_tok)
+        self.get_success(
+            self.account_data_handler.add_account_data_to_room(
+                user_id=user1_id,
+                room_id=room_id5,
+                account_data_type="org.matrix.roorarraz",
+                content={"roo": "rar"},
+            )
+        )
+
+        room_id_to_human_name_map = {
+            room_id1: "room1",
+            room_id2: "room2",
+            room_id3: "room3",
+            room_id4: "room4",
+            room_id5: "room5",
+        }
+
+        # Mix lists and rooms
+        sync_body = {
+            "lists": {
+                # We expect this list range to include room5 and room4
+                "foo-list": {
+                    "ranges": [[0, 1]],
+                    "required_state": [],
+                    "timeline_limit": 0,
+                },
+                # We expect this list range to include room5, room4, room3
+                "bar-list": {
+                    "ranges": [[0, 2]],
+                    "required_state": [],
+                    "timeline_limit": 0,
+                },
+            },
+            "room_subscriptions": {
+                room_id1: {
+                    "required_state": [],
+                    "timeline_limit": 0,
+                }
+            },
+            "extensions": {
+                "account_data": {
+                    "enabled": True,
+                    "lists": ["foo-list", "non-existent-list"],
+                    "rooms": [room_id1, room_id2, "!non-existent-room"],
+                }
+            },
+        }
+        response_body, _ = self.do_sync(sync_body, tok=user1_tok)
+
+        # room1: ✅ Requested via `rooms` and a room subscription exists
+        # room2: ❌ Requested via `rooms` but not in the response (from lists or room subscriptions)
+        # room3: ❌ Not requested
+        # room4: ✅ Shows up because requested via `lists` and list exists in the response
+        # room5: ✅ Shows up because requested via `lists` and list exists in the response
+        self.assertIncludes(
+            {
+                room_id_to_human_name_map[room_id]
+                for room_id in response_body["extensions"]["account_data"]
+                .get("rooms")
+                .keys()
+            },
+            {"room1", "room4", "room5"},
+            exact=True,
+        )
+
+        # Try wildcards (this is the default)
+        sync_body = {
+            "lists": {
+                # We expect this list range to include room5 and room4
+                "foo-list": {
+                    "ranges": [[0, 1]],
+                    "required_state": [],
+                    "timeline_limit": 0,
+                },
+                # We expect this list range to include room5, room4, room3
+                "bar-list": {
+                    "ranges": [[0, 2]],
+                    "required_state": [],
+                    "timeline_limit": 0,
+                },
+            },
+            "room_subscriptions": {
+                room_id1: {
+                    "required_state": [],
+                    "timeline_limit": 0,
+                }
+            },
+            "extensions": {
+                "account_data": {
+                    "enabled": True,
+                    # "lists": ["*"],
+                    # "rooms": ["*"],
+                }
+            },
+        }
+        response_body, _ = self.do_sync(sync_body, tok=user1_tok)
+
+        # room1: ✅ Shows up because of default `rooms` wildcard and is in one of the room subscriptions
+        # room2: ❌ Not requested
+        # room3: ✅ Shows up because of default `lists` wildcard and is in a list
+        # room4: ✅ Shows up because of default `lists` wildcard and is in a list
+        # room5: ✅ Shows up because of default `lists` wildcard and is in a list
+        self.assertIncludes(
+            {
+                room_id_to_human_name_map[room_id]
+                for room_id in response_body["extensions"]["account_data"]
+                .get("rooms")
+                .keys()
+            },
+            {"room1", "room3", "room4", "room5"},
+            exact=True,
+        )
+
+        # Empty list will return nothing
+        sync_body = {
+            "lists": {
+                # We expect this list range to include room5 and room4
+                "foo-list": {
+                    "ranges": [[0, 1]],
+                    "required_state": [],
+                    "timeline_limit": 0,
+                },
+                # We expect this list range to include room5, room4, room3
+                "bar-list": {
+                    "ranges": [[0, 2]],
+                    "required_state": [],
+                    "timeline_limit": 0,
+                },
+            },
+            "room_subscriptions": {
+                room_id1: {
+                    "required_state": [],
+                    "timeline_limit": 0,
+                }
+            },
+            "extensions": {
+                "account_data": {
+                    "enabled": True,
+                    "lists": [],
+                    "rooms": [],
+                }
+            },
+        }
+        response_body, _ = self.do_sync(sync_body, tok=user1_tok)
+
+        # room1: ❌ Not requested
+        # room2: ❌ Not requested
+        # room3: ❌ Not requested
+        # room4: ❌ Not requested
+        # room5: ❌ Not requested
+        self.assertIncludes(
+            {
+                room_id_to_human_name_map[room_id]
+                for room_id in response_body["extensions"]["account_data"]
+                .get("rooms")
+                .keys()
+            },
+            set(),
+            exact=True,
+        )
+
+        # Try wildcard and none
+        sync_body = {
+            "lists": {
+                # We expect this list range to include room5 and room4
+                "foo-list": {
+                    "ranges": [[0, 1]],
+                    "required_state": [],
+                    "timeline_limit": 0,
+                },
+                # We expect this list range to include room5, room4, room3
+                "bar-list": {
+                    "ranges": [[0, 2]],
+                    "required_state": [],
+                    "timeline_limit": 0,
+                },
+            },
+            "room_subscriptions": {
+                room_id1: {
+                    "required_state": [],
+                    "timeline_limit": 0,
+                }
+            },
+            "extensions": {
+                "account_data": {
+                    "enabled": True,
+                    "lists": ["*"],
+                    "rooms": [],
+                }
+            },
+        }
+        response_body, _ = self.do_sync(sync_body, tok=user1_tok)
+
+        # room1: ❌ Not requested
+        # room2: ❌ Not requested
+        # room3: ✅ Shows up because of default `lists` wildcard and is in a list
+        # room4: ✅ Shows up because of default `lists` wildcard and is in a list
+        # room5: ✅ Shows up because of default `lists` wildcard and is in a list
+        self.assertIncludes(
+            {
+                room_id_to_human_name_map[room_id]
+                for room_id in response_body["extensions"]["account_data"]
+                .get("rooms")
+                .keys()
+            },
+            {"room3", "room4", "room5"},
+            exact=True,
+        )
+
+    def test_wait_for_new_data(self) -> None:
+        """
+        Test to make sure that the Sliding Sync request waits for new data to arrive.
+
+        (Only applies to incremental syncs with a `timeout` specified)
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+        user2_id = self.register_user("user2", "pass")
+        user2_tok = self.login(user2_id, "pass")
+
+        room_id = self.helper.create_room_as(user2_id, tok=user2_tok)
+        self.helper.join(room_id, user1_id, tok=user1_tok)
+
+        sync_body = {
+            "lists": {},
+            "extensions": {
+                "account_data": {
+                    "enabled": True,
+                }
+            },
+        }
+        _, from_token = self.do_sync(sync_body, tok=user1_tok)
+
+        # Make an incremental Sliding Sync request with the account_data extension enabled
+        channel = self.make_request(
+            "POST",
+            self.sync_endpoint + f"?timeout=10000&pos={from_token}",
+            content=sync_body,
+            access_token=user1_tok,
+            await_result=False,
+        )
+        # Block for 5 seconds to make sure we are `notifier.wait_for_events(...)`
+        with self.assertRaises(TimedOutException):
+            channel.await_result(timeout_ms=5000)
+        # Bump the global account data to trigger new results
+        self.get_success(
+            self.account_data_handler.add_account_data_for_user(
+                user1_id,
+                "org.matrix.foobarbaz",
+                {"foo": "bar"},
+            )
+        )
+        # Should respond before the 10 second timeout
+        channel.await_result(timeout_ms=3000)
+        self.assertEqual(channel.code, 200, channel.json_body)
+
+        # We should see the global account data update
+        self.assertIncludes(
+            {
+                global_event["type"]
+                for global_event in channel.json_body["extensions"]["account_data"].get(
+                    "global"
+                )
+            },
+            {"org.matrix.foobarbaz"},
+            exact=True,
+        )
+        self.assertIncludes(
+            channel.json_body["extensions"]["account_data"].get("rooms").keys(),
+            set(),
+            exact=True,
+        )
+
+    def test_wait_for_new_data_timeout(self) -> None:
+        """
+        Test to make sure that the Sliding Sync request waits for new data to arrive but
+        no data ever arrives so we timeout. We're also making sure that the default data
+        from the account_data extension doesn't trigger a false-positive for new data.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+
+        sync_body = {
+            "lists": {},
+            "extensions": {
+                "account_data": {
+                    "enabled": True,
+                }
+            },
+        }
+        _, from_token = self.do_sync(sync_body, tok=user1_tok)
+
+        # Make the Sliding Sync request
+        channel = self.make_request(
+            "POST",
+            self.sync_endpoint + f"?timeout=10000&pos={from_token}",
+            content=sync_body,
+            access_token=user1_tok,
+            await_result=False,
+        )
+        # Block for 5 seconds to make sure we are `notifier.wait_for_events(...)`
+        with self.assertRaises(TimedOutException):
+            channel.await_result(timeout_ms=5000)
+        # Wake-up `notifier.wait_for_events(...)` that will cause us test
+        # `SlidingSyncResult.__bool__` for new results.
+        self._bump_notifier_wait_for_events(user1_id)
+        # Block for a little bit more to ensure we don't see any new results.
+        with self.assertRaises(TimedOutException):
+            channel.await_result(timeout_ms=4000)
+        # Wait for the sync to complete (wait for the rest of the 10 second timeout,
+        # 5000 + 4000 + 1200 > 10000)
+        channel.await_result(timeout_ms=1200)
+        self.assertEqual(channel.code, 200, channel.json_body)
+
+        self.assertIsNotNone(
+            channel.json_body["extensions"]["account_data"].get("global")
+        )
+        self.assertIsNotNone(
+            channel.json_body["extensions"]["account_data"].get("rooms")
+        )