diff --git a/changelog.d/9235.bugfix b/changelog.d/9235.bugfix
new file mode 100644
index 000000000000..7809c8673bb7
--- /dev/null
+++ b/changelog.d/9235.bugfix
@@ -0,0 +1 @@
+Fix a bug in the `make_room_admin` admin API where it failed if the admin with the greatest power level was not in the room. Contributed by Pankaj Yadav.
diff --git a/synapse/rest/admin/rooms.py b/synapse/rest/admin/rooms.py
index da1499cab3b3..f14915d47ee7 100644
--- a/synapse/rest/admin/rooms.py
+++ b/synapse/rest/admin/rooms.py
@@ -431,7 +431,17 @@ async def on_POST(self, request, room_identifier):
             if not admin_users:
                 raise SynapseError(400, "No local admin user in room")
 
-            admin_user_id = admin_users[-1]
+            admin_user_id = None
+
+            for admin_user in reversed(admin_users):
+                if room_state.get((EventTypes.Member, admin_user)):
+                    admin_user_id = admin_user
+                    break
+
+            if not admin_user_id:
+                raise SynapseError(
+                    400, "No local admin user in room",
+                )
 
             pl_content = power_levels.content
         else: