diff --git a/changelog.d/5966.bugfix b/changelog.d/5966.bugfix
new file mode 100644
index 0000000000..b8ef5a7819
--- /dev/null
+++ b/changelog.d/5966.bugfix
@@ -0,0 +1 @@
+Fix admin API for listing media in a room not being available with an external media repo.
diff --git a/synapse/rest/admin/__init__.py b/synapse/rest/admin/__init__.py
index b4761adaed..81b6bd8816 100644
--- a/synapse/rest/admin/__init__.py
+++ b/synapse/rest/admin/__init__.py
@@ -41,7 +41,7 @@ from synapse.rest.admin._base import (
     assert_user_is_admin,
     historical_admin_path_patterns,
 )
-from synapse.rest.admin.media import register_servlets_for_media_repo
+from synapse.rest.admin.media import ListMediaInRoom, register_servlets_for_media_repo
 from synapse.rest.admin.purge_room_servlet import PurgeRoomServlet
 from synapse.rest.admin.server_notice_servlet import SendServerNoticeServlet
 from synapse.rest.admin.users import UserAdminServlet
@@ -761,9 +761,12 @@ def register_servlets_for_client_rest_resource(hs, http_server):
     DeleteGroupAdminRestServlet(hs).register(http_server)
     AccountValidityRenewServlet(hs).register(http_server)
 
-    # Load the media repo ones if we're using them.
+    # Load the media repo ones if we're using them. Otherwise load the servlets which
+    # don't need a media repo (typically readonly admin APIs).
     if hs.config.can_load_media_repo:
         register_servlets_for_media_repo(hs, http_server)
+    else:
+        ListMediaInRoom(hs).register(http_server)
 
     # don't add more things here: new servlets should only be exposed on
     # /_synapse/admin so should not go here. Instead register them in AdminRestResource.