diff --git a/homeassistant/components/websocket_api/messages.py b/homeassistant/components/websocket_api/messages.py
index 3d85f984e9a..e5fd5626302 100644
--- a/homeassistant/components/websocket_api/messages.py
+++ b/homeassistant/components/websocket_api/messages.py
@@ -180,7 +180,10 @@ def _state_diff(
             if old_attributes.get(key) != value:
                 additions.setdefault(COMPRESSED_STATE_ATTRIBUTES, {})[key] = value
         if removed := set(old_attributes).difference(new_attributes):
-            diff[STATE_DIFF_REMOVALS] = {COMPRESSED_STATE_ATTRIBUTES: removed}
+            # sets are not JSON serializable by default so we convert to list
+            # here if there are any values to avoid jumping into the json_encoder_default
+            # for every state diff with a removed attribute
+            diff[STATE_DIFF_REMOVALS] = {COMPRESSED_STATE_ATTRIBUTES: list(removed)}
     return {ENTITY_EVENT_CHANGE: {new_state.entity_id: diff}}
 
 
diff --git a/tests/components/websocket_api/test_messages.py b/tests/components/websocket_api/test_messages.py
index d2102b651b7..6aafb9f2685 100644
--- a/tests/components/websocket_api/test_messages.py
+++ b/tests/components/websocket_api/test_messages.py
@@ -222,6 +222,21 @@ async def test_state_diff_event(hass: HomeAssistant) -> None:
         }
     }
 
+    hass.states.async_set("light.window", "green", {}, context=new_context)
+    await hass.async_block_till_done()
+    last_state_event: Event = state_change_events[-1]
+    new_state: State = last_state_event.data["new_state"]
+    message = _state_diff_event(last_state_event)
+
+    assert message == {
+        "c": {
+            "light.window": {
+                "+": {"lc": new_state.last_changed.timestamp(), "s": "green"},
+                "-": {"a": ["new"]},
+            }
+        }
+    }
+
 
 async def test_message_to_json(caplog: pytest.LogCaptureFixture) -> None:
     """Test we can serialize websocket messages."""