diff --git a/changelog.d/18675.feature b/changelog.d/18675.feature
new file mode 100644
index 000000000..7ff244580
--- /dev/null
+++ b/changelog.d/18675.feature
@@ -0,0 +1 @@
+Include `event_id` when getting state with `?format=event`. Contributed by @tulir @ Beeper.
diff --git a/synapse/rest/client/room.py b/synapse/rest/client/room.py
index 4600a8777..6b0deda0d 100644
--- a/synapse/rest/client/room.py
+++ b/synapse/rest/client/room.py
@@ -44,7 +44,11 @@ from synapse.api.errors import (
     UnredactedContentDeletedError,
 )
 from synapse.api.filtering import Filter
-from synapse.events.utils import SerializeEventConfig, format_event_for_client_v2
+from synapse.events.utils import (
+    SerializeEventConfig,
+    format_event_for_client_v2,
+    serialize_event,
+)
 from synapse.http.server import HttpServer
 from synapse.http.servlet import (
     ResolveRoomIdMixin,
@@ -198,6 +202,7 @@ class RoomStateEventRestServlet(RestServlet):
         self.message_handler = hs.get_message_handler()
         self.delayed_events_handler = hs.get_delayed_events_handler()
         self.auth = hs.get_auth()
+        self.clock = hs.get_clock()
         self._max_event_delay_ms = hs.config.server.max_event_delay_ms
         self._spam_checker_module_callbacks = hs.get_module_api_callbacks().spam_checker
 
@@ -268,7 +273,14 @@ class RoomStateEventRestServlet(RestServlet):
             raise SynapseError(404, "Event not found.", errcode=Codes.NOT_FOUND)
 
         if format == "event":
-            event = format_event_for_client_v2(data.get_dict())
+            event = serialize_event(
+                data,
+                self.clock.time_msec(),
+                config=SerializeEventConfig(
+                    event_format=format_event_for_client_v2,
+                    requester=requester,
+                ),
+            )
             return 200, event
         elif format == "content":
             return 200, data.get_dict()["content"]
diff --git a/tests/rest/client/test_rooms.py b/tests/rest/client/test_rooms.py
index 8a6e6f118..48d33b8e1 100644
--- a/tests/rest/client/test_rooms.py
+++ b/tests/rest/client/test_rooms.py
@@ -552,6 +552,51 @@ class RoomStateTestCase(RoomBase):
         self.assertEqual(HTTPStatus.OK, channel.code, msg=channel.result["body"])
         self.assertEqual(channel.json_body, {"membership": "join"})
 
+    def test_get_state_format_content(self) -> None:
+        """Test response of a `/rooms/$room_id/state/$event_type?format=content` request."""
+        room_id = self.helper.create_room_as(self.user_id)
+        channel1 = self.make_request(
+            "GET",
+            "/rooms/%s/state/m.room.member/%s?format=content"
+            % (
+                room_id,
+                self.user_id,
+            ),
+        )
+        self.assertEqual(channel1.code, HTTPStatus.OK, channel1.json_body)
+        self.assertEqual(channel1.json_body, {"membership": "join"})
+        channel2 = self.make_request(
+            "GET",
+            "/rooms/%s/state/m.room.member/%s"
+            % (
+                room_id,
+                self.user_id,
+            ),
+        )
+        self.assertEqual(channel2.code, HTTPStatus.OK, channel2.json_body)
+        # "content" is the default format.
+        self.assertEqual(channel1.json_body, channel2.json_body)
+
+    def test_get_state_format_event(self) -> None:
+        """Test response of a `/rooms/$room_id/state/$event_type?format=event` request."""
+        room_id = self.helper.create_room_as(self.user_id)
+        channel = self.make_request(
+            "GET",
+            "/rooms/%s/state/m.room.member/%s?format=event"
+            % (
+                room_id,
+                self.user_id,
+            ),
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK, channel.json_body)
+        self.assertEqual(channel.json_body["content"], {"membership": "join"})
+        self.assertEqual(channel.json_body["room_id"], room_id)
+        self.assertRegex(channel.json_body["event_id"], r"\$.+")
+        self.assertEqual(channel.json_body["type"], "m.room.member")
+        self.assertEqual(channel.json_body["sender"], self.user_id)
+        self.assertEqual(channel.json_body["state_key"], self.user_id)
+        self.assertTrue(type(channel.json_body["origin_server_ts"]) is int)
+
 
 class RoomsMemberListTestCase(RoomBase):
     """Tests /rooms/$room_id/members/list REST events."""