diff --git a/changelog.d/16973.bugfix b/changelog.d/16973.bugfix
new file mode 100644
index 0000000000000000000000000000000000000000..a0e3a123049286b8acddd37f9538564d19ed3680
--- /dev/null
+++ b/changelog.d/16973.bugfix
@@ -0,0 +1 @@
+Fix joining remote rooms when a module uses the `on_new_event` callback. This callback may now pass partial state events instead of the full state for remote rooms.
diff --git a/docs/modules/third_party_rules_callbacks.md b/docs/modules/third_party_rules_callbacks.md
index d06bff25eb1aaa19db4792fbb9d23d59b13c6e9c..b97e28db110e07a03ae3ef645da6b16bc04b7700 100644
--- a/docs/modules/third_party_rules_callbacks.md
+++ b/docs/modules/third_party_rules_callbacks.md
@@ -142,6 +142,10 @@ Called after sending an event into a room. The module is passed the event, as we
 as the state of the room _after_ the event. This means that if the event is a state event,
 it will be included in this state.
 
+The state map may not be complete if Synapse hasn't yet loaded the full state
+of the room. This can happen for events in rooms that were just joined from
+a remote server.
+
 Note that this callback is called when the event has already been processed and stored
 into the room, which means this callback cannot be used to deny persisting the event. To
 deny an incoming event, see [`check_event_for_spam`](spam_checker_callbacks.md#check_event_for_spam) instead.
diff --git a/synapse/module_api/callbacks/third_party_event_rules_callbacks.py b/synapse/module_api/callbacks/third_party_event_rules_callbacks.py
index 7a3255aeef6d7d1e6076eed1718b551b118da3e2..9f7a04372deb003fdd5f3d7cfa49e34ed6be3de6 100644
--- a/synapse/module_api/callbacks/third_party_event_rules_callbacks.py
+++ b/synapse/module_api/callbacks/third_party_event_rules_callbacks.py
@@ -366,7 +366,7 @@ class ThirdPartyEventRulesModuleApiCallbacks:
         if len(self._check_threepid_can_be_invited_callbacks) == 0:
             return True
 
-        state_events = await self._get_state_map_for_room(room_id)
+        state_events = await self._storage_controllers.state.get_current_state(room_id)
 
         for callback in self._check_threepid_can_be_invited_callbacks:
             try:
@@ -399,7 +399,7 @@ class ThirdPartyEventRulesModuleApiCallbacks:
         if len(self._check_visibility_can_be_modified_callbacks) == 0:
             return True
 
-        state_events = await self._get_state_map_for_room(room_id)
+        state_events = await self._storage_controllers.state.get_current_state(room_id)
 
         for callback in self._check_visibility_can_be_modified_callbacks:
             try:
@@ -427,7 +427,13 @@ class ThirdPartyEventRulesModuleApiCallbacks:
             return
 
         event = await self.store.get_event(event_id)
-        state_events = await self._get_state_map_for_room(event.room_id)
+
+        # We *don't* want to wait for the full state here, because waiting for full
+        # state will persist event, which in turn will call this method.
+        # This would end up in a deadlock.
+        state_events = await self._storage_controllers.state.get_current_state(
+            event.room_id, await_full_state=False
+        )
 
         for callback in self._on_new_event_callbacks:
             try:
@@ -490,17 +496,6 @@ class ThirdPartyEventRulesModuleApiCallbacks:
                 )
         return True
 
-    async def _get_state_map_for_room(self, room_id: str) -> StateMap[EventBase]:
-        """Given a room ID, return the state events of that room.
-
-        Args:
-            room_id: The ID of the room.
-
-        Returns:
-            A dict mapping (event type, state key) to state event.
-        """
-        return await self._storage_controllers.state.get_current_state(room_id)
-
     async def on_profile_update(
         self, user_id: str, new_profile: ProfileInfo, by_admin: bool, deactivation: bool
     ) -> None:
diff --git a/synapse/storage/controllers/state.py b/synapse/storage/controllers/state.py
index 99666d79a9b33953e50867c0ef59ca121d6dd25c..22d93a561c62932d069c6e524dce01b90ee4a740 100644
--- a/synapse/storage/controllers/state.py
+++ b/synapse/storage/controllers/state.py
@@ -562,10 +562,15 @@ class StateStorageController:
     @trace
     @tag_args
     async def get_current_state(
-        self, room_id: str, state_filter: Optional[StateFilter] = None
+        self,
+        room_id: str,
+        state_filter: Optional[StateFilter] = None,
+        await_full_state: bool = True,
     ) -> StateMap[EventBase]:
         """Same as `get_current_state_ids` but also fetches the events"""
-        state_map_ids = await self.get_current_state_ids(room_id, state_filter)
+        state_map_ids = await self.get_current_state_ids(
+            room_id, state_filter, await_full_state
+        )
 
         event_map = await self.stores.main.get_events(list(state_map_ids.values()))