diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index 62ae371ca3..909792095c 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -1720,10 +1720,10 @@ class PersistEventsStore:
             txn.execute_batch(
                 f"""
                 INSERT INTO sliding_sync_membership_snapshots
-                    (room_id, user_id, membership_event_id, membership, event_stream_ordering
+                    (room_id, user_id, sender, membership_event_id, membership, event_stream_ordering
                     {("," + ", ".join(insert_keys)) if insert_keys else ""})
                 VALUES (
-                    ?, ?, ?, ?, ?
+                    ?, ?, ?, ?, ?, ?
                     {("," + ", ".join("?" for _ in insert_values)) if insert_values else ""}
                 )
                 ON CONFLICT (room_id, user_id)
@@ -1737,6 +1737,7 @@ class PersistEventsStore:
                     [
                         room_id,
                         membership_info.user_id,
+                        membership_info.sender,
                         membership_info.membership_event_id,
                         membership_info.membership,
                         membership_info.membership_event_stream_ordering,
@@ -2693,6 +2694,7 @@ class PersistEventsStore:
                     raw_stripped_state_events = knock_room_state
 
                 insert_values = {
+                    "sender": event.sender,
                     "membership_event_id": event.event_id,
                     "membership": event.membership,
                     "event_stream_ordering": event.internal_metadata.stream_ordering,
diff --git a/synapse/storage/databases/main/events_bg_updates.py b/synapse/storage/databases/main/events_bg_updates.py
index 5d04955bf1..a2bfaabafd 100644
--- a/synapse/storage/databases/main/events_bg_updates.py
+++ b/synapse/storage/databases/main/events_bg_updates.py
@@ -1631,6 +1631,8 @@ class EventsBackgroundUpdatesStore(StreamWorkerStore, StateDeltasStore, SQLBaseS
             )
 
         def _backfill_table_txn(txn: LoggingTransaction) -> None:
+            # Handle updating the `sliding_sync_joined_rooms` table
+            #
             last_successful_room_id: Optional[str] = None
             for room_id, insert_map in joined_room_updates.items():
                 (
@@ -1965,9 +1967,12 @@ class EventsBackgroundUpdatesStore(StreamWorkerStore, StateDeltasStore, SQLBaseS
             )
 
         def _backfill_table_txn(txn: LoggingTransaction) -> None:
+            # Handle updating the `sliding_sync_membership_snapshots` table
+            #
             for key, insert_map in to_insert_membership_snapshots.items():
                 room_id, user_id = key
                 membership_info = to_insert_membership_infos[key]
+                sender = membership_info.sender
                 membership_event_id = membership_info.membership_event_id
                 membership = membership_info.membership
                 membership_event_stream_ordering = (
@@ -1983,10 +1988,10 @@ class EventsBackgroundUpdatesStore(StreamWorkerStore, StateDeltasStore, SQLBaseS
                 txn.execute(
                     f"""
                     INSERT INTO sliding_sync_membership_snapshots
-                        (room_id, user_id, membership_event_id, membership, event_stream_ordering
+                        (room_id, user_id, sender, membership_event_id, membership, event_stream_ordering
                         {("," + ", ".join(insert_keys)) if insert_keys else ""})
                     VALUES (
-                        ?, ?, ?, ?, ?
+                        ?, ?, ?, ?, ?, ?
                         {("," + ", ".join("?" for _ in insert_values)) if insert_values else ""}
                     )
                     ON CONFLICT (room_id, user_id)
@@ -1995,6 +2000,7 @@ class EventsBackgroundUpdatesStore(StreamWorkerStore, StateDeltasStore, SQLBaseS
                     [
                         room_id,
                         user_id,
+                        sender,
                         membership_event_id,
                         membership,
                         membership_event_stream_ordering,
diff --git a/synapse/storage/schema/main/delta/87/01_sliding_sync_memberships.sql b/synapse/storage/schema/main/delta/87/01_sliding_sync_memberships.sql
index 16b3f84c3d..5fac6af619 100644
--- a/synapse/storage/schema/main/delta/87/01_sliding_sync_memberships.sql
+++ b/synapse/storage/schema/main/delta/87/01_sliding_sync_memberships.sql
@@ -60,6 +60,8 @@ CREATE TABLE IF NOT EXISTS sliding_sync_joined_rooms(
 CREATE TABLE IF NOT EXISTS sliding_sync_membership_snapshots(
     room_id TEXT NOT NULL REFERENCES rooms(room_id),
     user_id TEXT NOT NULL,
+    -- Useful to be able to tell leaves from kicks (where the `user_id` is different from the `sender`)
+    sender TEXT NOT NULL,
     membership_event_id TEXT NOT NULL REFERENCES events(event_id),
     membership TEXT NOT NULL,
     -- `stream_ordering` of the `membership_event_id`