diff --git a/synapse/config/user_directory.py b/synapse/config/user_directory.py
index 38e8947843..9b0ae91289 100644
--- a/synapse/config/user_directory.py
+++ b/synapse/config/user_directory.py
@@ -23,11 +23,15 @@ class UserDirectoryConfig(Config):
 
     def read_config(self, config):
         self.user_directory_search_all_users = False
+        self.user_directory_defer_to_id_server = None
         user_directory_config = config.get("user_directory", None)
         if user_directory_config:
             self.user_directory_search_all_users = (
                 user_directory_config.get("search_all_users", False)
             )
+            self.user_directory_defer_to_id_server = (
+                user_directory_config.get("defer_to_id_server", None)
+            )
 
     def default_config(self, config_dir_path, server_name, **kwargs):
         return """
@@ -41,4 +45,9 @@ class UserDirectoryConfig(Config):
         #
         #user_directory:
         #   search_all_users: false
+        #
+        #   If this is set, user search will be delegated to this ID server instead
+        #   of synapse performing the search itself.
+        #   This is an experimental API.
+        #   defer_to_id_server: id.example.com
         """
diff --git a/synapse/rest/client/v2_alpha/user_directory.py b/synapse/rest/client/v2_alpha/user_directory.py
index 2d4a43c353..9b1a019978 100644
--- a/synapse/rest/client/v2_alpha/user_directory.py
+++ b/synapse/rest/client/v2_alpha/user_directory.py
@@ -16,6 +16,7 @@
 import logging
 
 from twisted.internet import defer
+from signedjson.sign import sign_json
 
 from synapse.api.errors import SynapseError
 from synapse.http.servlet import RestServlet, parse_json_object_from_request
@@ -36,6 +37,7 @@ class UserDirectorySearchRestServlet(RestServlet):
         self.hs = hs
         self.auth = hs.get_auth()
         self.user_directory_handler = hs.get_user_directory_handler()
+        self.http_client = hs.get_simple_http_client()
 
     @defer.inlineCallbacks
     def on_POST(self, request):
@@ -60,6 +62,14 @@ class UserDirectorySearchRestServlet(RestServlet):
 
         body = parse_json_object_from_request(request)
 
+        if self.hs.config.user_directory_defer_to_id_server:
+            signed_body = sign_json(body, self.hs.hostname, self.hs.config.signing_key[0])
+            url = "http://%s/_matrix/identity/api/v1/user_directory/search" % (
+                self.hs.config.user_directory_defer_to_id_server,
+            )
+            resp = yield self.http_client.post_json_get_json(url, signed_body)
+            defer.returnValue((200, resp))
+
         limit = body.get("limit", 10)
         limit = min(limit, 50)