diff --git a/changelog.d/11690.misc b/changelog.d/11690.misc
new file mode 100644
index 0000000000..76cd286862
--- /dev/null
+++ b/changelog.d/11690.misc
@@ -0,0 +1 @@
+Update the `/capabilities` response to include whether support for [MSC3440](https://github.com/matrix-org/matrix-doc/pull/3440) is available.
diff --git a/synapse/rest/client/capabilities.py b/synapse/rest/client/capabilities.py
index 2a3e24ae7e..5c0e3a5680 100644
--- a/synapse/rest/client/capabilities.py
+++ b/synapse/rest/client/capabilities.py
@@ -73,6 +73,9 @@ class CapabilitiesRestServlet(RestServlet):
                 "enabled": self.config.registration.enable_3pid_changes
             }
 
+        if self.config.experimental.msc3440_enabled:
+            response["capabilities"]["io.element.thread"] = {"enabled": True}
+
         return 200, response