diff --git a/changelog.d/14064.bugfix b/changelog.d/14064.bugfix
new file mode 100644
index 0000000000..cce6ef3b71
--- /dev/null
+++ b/changelog.d/14064.bugfix
@@ -0,0 +1 @@
+ Fix a long-standing bug where Synapse would accidentally include extra information in the response to [`PUT /_matrix/federation/v2/invite/{roomId}/{eventId}`](https://spec.matrix.org/v1.4/server-server-api/#put_matrixfederationv2inviteroomideventid).
\ No newline at end of file
diff --git a/synapse/federation/transport/server/federation.py b/synapse/federation/transport/server/federation.py
index 6f11138b57..205fd16daa 100644
--- a/synapse/federation/transport/server/federation.py
+++ b/synapse/federation/transport/server/federation.py
@@ -499,6 +499,11 @@ class FederationV2InviteServlet(BaseFederationServerServlet):
         result = await self.handler.on_invite_request(
             origin, event, room_version_id=room_version
         )
+
+        # We only store invite_room_state for internal use, so remove it before
+        # returning the event to the remote homeserver.
+        result["event"].get("unsigned", {}).pop("invite_room_state", None)
+
         return 200, result