diff --git a/changelog.d/16403.bugfix b/changelog.d/16403.bugfix
new file mode 100644
index 0000000000..453c975a63
--- /dev/null
+++ b/changelog.d/16403.bugfix
@@ -0,0 +1 @@
+Remove legacy unspecced `knock_state_events` field returned in some responses.
diff --git a/synapse/federation/federation_client.py b/synapse/federation/federation_client.py
index c8bc46415d..1a7fa175ec 100644
--- a/synapse/federation/federation_client.py
+++ b/synapse/federation/federation_client.py
@@ -1402,7 +1402,7 @@ class FederationClient(FederationBase):
             The remote homeserver return some state from the room. The response
             dictionary is in the form:
 
-            {"knock_state_events": [<state event dict>, ...]}
+            {"knock_room_state": [<state event dict>, ...]}
 
             The list of state events may be empty.
 
@@ -1429,7 +1429,7 @@ class FederationClient(FederationBase):
             The remote homeserver can optionally return some state from the room. The response
             dictionary is in the form:
 
-            {"knock_state_events": [<state event dict>, ...]}
+            {"knock_room_state": [<state event dict>, ...]}
 
             The list of state events may be empty.
         """
diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index ec8e770430..6ac8d16095 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -850,14 +850,7 @@ class FederationServer(FederationBase):
                 context, self._room_prejoin_state_types
             )
         )
-        return {
-            "knock_room_state": stripped_room_state,
-            # Since v1.37, Synapse incorrectly used "knock_state_events" for this field.
-            # Thus, we also populate a 'knock_state_events' with the same content to
-            # support old instances.
-            # See https://github.com/matrix-org/synapse/issues/14088.
-            "knock_state_events": stripped_room_state,
-        }
+        return {"knock_room_state": stripped_room_state}
 
     async def _on_send_membership_event(
         self, origin: str, content: JsonDict, membership_type: str, room_id: str
diff --git a/synapse/federation/transport/client.py b/synapse/federation/transport/client.py
index b5e4b2680e..fab4800717 100644
--- a/synapse/federation/transport/client.py
+++ b/synapse/federation/transport/client.py
@@ -431,7 +431,7 @@ class TransportLayerClient:
             The remote homeserver can optionally return some state from the room. The response
             dictionary is in the form:
 
-            {"knock_state_events": [<state event dict>, ...]}
+            {"knock_room_state": [<state event dict>, ...]}
 
             The list of state events may be empty.
         """
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 29cd45550a..807a0867cc 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -868,19 +868,10 @@ class FederationHandler:
         # This is a bit of a hack and is cribbing off of invites. Basically we
         # store the room state here and retrieve it again when this event appears
         # in the invitee's sync stream. It is stripped out for all other local users.
-        stripped_room_state = (
-            knock_response.get("knock_room_state")
-            # Since v1.37, Synapse incorrectly used "knock_state_events" for this field.
-            # Thus, we also check for a 'knock_state_events' to support old instances.
-            # See https://github.com/matrix-org/synapse/issues/14088.
-            or knock_response.get("knock_state_events")
-        )
+        stripped_room_state = knock_response.get("knock_room_state")
 
         if stripped_room_state is None:
-            raise KeyError(
-                "Missing 'knock_room_state' (or legacy 'knock_state_events') field in "
-                "send_knock response"
-            )
+            raise KeyError("Missing 'knock_room_state' field in send_knock response")
 
         event.unsigned["knock_room_state"] = stripped_room_state
 
diff --git a/tests/federation/transport/test_knocking.py b/tests/federation/transport/test_knocking.py
index 3f42f79f26..b63ef3d4ed 100644
--- a/tests/federation/transport/test_knocking.py
+++ b/tests/federation/transport/test_knocking.py
@@ -308,7 +308,7 @@ class FederationKnockingTestCase(
         self.assertEqual(200, channel.code, channel.result)
 
         # Check that we got the stripped room state in return
-        room_state_events = channel.json_body["knock_state_events"]
+        room_state_events = channel.json_body["knock_room_state"]
 
         # Validate the stripped room state events
         self.check_knock_room_state_against_room_state(