diff --git a/changelog.d/18582.bugfix b/changelog.d/18582.bugfix
new file mode 100644
index 000000000..7091bcc99
--- /dev/null
+++ b/changelog.d/18582.bugfix
@@ -0,0 +1 @@
+Improve performance of device deletion by adding missing index.
\ No newline at end of file
diff --git a/synapse/storage/databases/main/events_worker.py b/synapse/storage/databases/main/events_worker.py
index 3db4460f5..9cc049330 100644
--- a/synapse/storage/databases/main/events_worker.py
+++ b/synapse/storage/databases/main/events_worker.py
@@ -349,6 +349,19 @@ class EventsWorkerStore(SQLBaseStore):
             where_clause="type = 'm.room.member'",
         )
 
+        # Added to support efficient reverse lookups on the foreign key
+        # (user_id, device_id) when deleting devices.
+        # We already had a UNIQUE index on these 4 columns but out-of-order
+        # so replace that one.
+        self.db_pool.updates.register_background_index_update(
+            update_name="event_txn_id_device_id_txn_id2",
+            index_name="event_txn_id_device_id_txn_id2",
+            table="event_txn_id_device_id",
+            columns=("user_id", "device_id", "room_id", "txn_id"),
+            unique=True,
+            replaces_index="event_txn_id_device_id_txn_id",
+        )
+
     def get_un_partial_stated_events_token(self, instance_name: str) -> int:
         return (
             self._un_partial_stated_events_stream_id_gen.get_current_token_for_writer(
diff --git a/synapse/storage/schema/main/delta/92/07_event_txn_id_device_id_txn_id2.sql b/synapse/storage/schema/main/delta/92/07_event_txn_id_device_id_txn_id2.sql
new file mode 100644
index 000000000..b568fc04f
--- /dev/null
+++ b/synapse/storage/schema/main/delta/92/07_event_txn_id_device_id_txn_id2.sql
@@ -0,0 +1,15 @@
+--
+-- This file is licensed under the Affero General Public License (AGPL) version 3.
+--
+-- Copyright (C) 2025 New Vector, Ltd
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU Affero General Public License as
+-- published by the Free Software Foundation, either version 3 of the
+-- License, or (at your option) any later version.
+--
+-- See the GNU Affero General Public License for more details:
+-- <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+INSERT INTO background_updates (ordering, update_name, progress_json) VALUES
+  (9207, 'event_txn_id_device_id_txn_id2', '{}');