diff --git a/changelog.d/10212.feature b/changelog.d/10212.feature
new file mode 100644
index 000000000000..9c41140194b2
--- /dev/null
+++ b/changelog.d/10212.feature
@@ -0,0 +1 @@
+Implement "room knocking" as per [MSC2403](https://github.com/matrix-org/matrix-doc/pull/2403). Contributed by Sorunome and anoa.
\ No newline at end of file
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 0bfb25802af2..1b566dbf2dad 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -2086,8 +2086,6 @@ async def on_send_knock_request(
 
         context = await self.state_handler.compute_event_context(event)
 
-        await self._auth_and_persist_event(origin, event, context)
-
         event_allowed = await self.third_party_event_rules.check_event_allowed(
             event, context
         )
@@ -2097,6 +2095,8 @@ async def on_send_knock_request(
                 403, "This event is not allowed in this context", Codes.FORBIDDEN
             )
 
+        await self._auth_and_persist_event(origin, event, context)
+
         return context
 
     async def get_state_for_pdu(self, room_id: str, event_id: str) -> List[EventBase]: