diff --git a/synapse/rest/admin/__init__.py b/synapse/rest/admin/__init__.py
index f3c99663e84..1d527a96516 100644
--- a/synapse/rest/admin/__init__.py
+++ b/synapse/rest/admin/__init__.py
@@ -81,6 +81,7 @@
     RoomEventContextServlet,
     RoomMembersRestServlet,
     RoomMessagesRestServlet,
+    RoomParticipantsRestServlet,
     RoomRestServlet,
     RoomRestV2Servlet,
     RoomStateRestServlet,
@@ -320,6 +321,7 @@ def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     ListDestinationsRestServlet(hs).register(http_server)
     RoomMessagesRestServlet(hs).register(http_server)
     RoomTimestampToEventRestServlet(hs).register(http_server)
+    RoomParticipantsRestServlet(hs).register(http_server)
     UserReplaceMasterCrossSigningKeyRestServlet(hs).register(http_server)
     UserByExternalId(hs).register(http_server)
     UserByThreePid(hs).register(http_server)
diff --git a/synapse/rest/admin/rooms.py b/synapse/rest/admin/rooms.py
index 01f9de9ffa5..601c1f69542 100644
--- a/synapse/rest/admin/rooms.py
+++ b/synapse/rest/admin/rooms.py
@@ -989,3 +989,28 @@ async def on_GET(
             "event_id": event_id,
             "origin_server_ts": origin_server_ts,
         }
+
+
+class RoomParticipantsRestServlet(RestServlet):
+    """
+    This endpoint allows admins to fetch currently joined members of a room who
+    have also posted to the room.
+
+    GET /_synapse/admin/v1/rooms/{room_id}/participants
+    """
+
+    PATTERNS = admin_patterns("/rooms/(?P<room_id>[^/]*)/participants$")
+
+    def __init__(self, hs: "HomeServer"):
+        self._auth = hs.get_auth()
+        self._store = hs.get_datastores().main
+
+    async def on_GET(
+        self, request: SynapseRequest, room_id: str
+    ) -> Tuple[int, JsonDict]:
+        requester = await self._auth.get_user_by_req(request)
+        await assert_user_is_admin(self._auth, requester)
+
+        participants = await self._store.get_participants_in_room(room_id)
+
+        return HTTPStatus.OK, {"participants": list(participants)}
diff --git a/synapse/storage/databases/main/roommember.py b/synapse/storage/databases/main/roommember.py
index 50ed6a28bf0..007521bcb46 100644
--- a/synapse/storage/databases/main/roommember.py
+++ b/synapse/storage/databases/main/roommember.py
@@ -1606,6 +1606,34 @@ def _get_rooms_for_user_by_join_date_txn(
             from_ts,
         )
 
+    async def get_participants_in_room(self, room_id: str) -> FrozenSet[str]:
+        """
+        Return a list of all currently joined room members who have posted
+        in the given room.
+
+        Args:
+            room_id: room ID of the room to search in
+        """
+
+        def _get_participants_in_room_txn(
+            txn: LoggingTransaction, room_id: str
+        ) -> frozenset:
+            sql = """
+                SELECT DISTINCT c.state_key
+                FROM current_state_events AS c
+                INNER JOIN events AS e USING(room_id)
+                WHERE room_id = ?
+                    AND c.membership = 'join'
+                    AND e.type = 'm.room.message'
+                    AND c.state_key = e.sender
+            """
+            txn.execute(sql, (room_id,))
+            return frozenset([r[0] for r in txn])
+
+        return await self.db_pool.runInteraction(
+            "_get_participants_in_room_txn", _get_participants_in_room_txn, room_id
+        )
+
 
 class RoomMemberBackgroundUpdateStore(SQLBaseStore):
     def __init__(