diff --git a/federationapi/routing/send.go b/federationapi/routing/send.go
index dca6a7ef..a817e837 100644
--- a/federationapi/routing/send.go
+++ b/federationapi/routing/send.go
@@ -655,9 +655,14 @@ func checkAllowedByState(e *gomatrixserverlib.Event, stateEvents []*gomatrixserv
 	return gomatrixserverlib.Allowed(e, &authUsingState)
 }
 
+var processEventWithMissingStateMutexes = internal.NewMutexByRoom()
+
 func (t *txnReq) processEventWithMissingState(
 	ctx context.Context, e *gomatrixserverlib.Event, roomVersion gomatrixserverlib.RoomVersion,
 ) error {
+	processEventWithMissingStateMutexes.Lock(e.RoomID())
+	defer processEventWithMissingStateMutexes.Unlock(e.RoomID())
+
 	// We are missing the previous events for this events.
 	// This means that there is a gap in our view of the history of the
 	// room. There two ways that we can handle such a gap: