diff --git a/synapse/storage/client_ips.py b/synapse/storage/client_ips.py
index a03d1d6104..7b44dae0fc 100644
--- a/synapse/storage/client_ips.py
+++ b/synapse/storage/client_ips.py
@@ -48,6 +48,13 @@ class ClientIpStore(background_updates.BackgroundUpdateStore):
             columns=["user_id", "device_id", "last_seen"],
         )
 
+        self.register_background_index_update(
+            "user_ips_last_seen_index",
+            index_name="user_ips_last_seen",
+            table="user_ips",
+            columns=["user_id", "last_seen"],
+        )
+
         # (user_id, access_token, ip) -> (user_agent, device_id, last_seen)
         self._batch_row_update = {}
 
diff --git a/synapse/storage/schema/delta/48/add_user_ips_last_seen_index.sql b/synapse/storage/schema/delta/48/add_user_ips_last_seen_index.sql
new file mode 100644
index 0000000000..9248b0b24a
--- /dev/null
+++ b/synapse/storage/schema/delta/48/add_user_ips_last_seen_index.sql
@@ -0,0 +1,17 @@
+/* Copyright 2018 New Vector Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+INSERT into background_updates (update_name, progress_json)
+    VALUES ('user_ips_last_seen_index', '{}');