diff --git a/mem0/memory/main.py b/mem0/memory/main.py
index b68142529..70a4c0917 100644
--- a/mem0/memory/main.py
+++ b/mem0/memory/main.py
@@ -1289,7 +1289,7 @@ class Memory(MemoryBase):
             new_metadata["agent_id"] = existing_memory.payload["agent_id"]
         if "run_id" not in new_metadata and "run_id" in existing_memory.payload:
             new_metadata["run_id"] = existing_memory.payload["run_id"]
-        if "actor_id" not in new_metadata and "actor_id" in existing_memory.payload:
+        if "actor_id" in existing_memory.payload:
             new_metadata["actor_id"] = existing_memory.payload["actor_id"]
         if "role" not in new_metadata and "role" in existing_memory.payload:
             new_metadata["role"] = existing_memory.payload["role"]
@@ -2440,7 +2440,7 @@ class AsyncMemory(MemoryBase):
         if "run_id" not in new_metadata and "run_id" in existing_memory.payload:
             new_metadata["run_id"] = existing_memory.payload["run_id"]
 
-        if "actor_id" not in new_metadata and "actor_id" in existing_memory.payload:
+        if "actor_id" in existing_memory.payload:
             new_metadata["actor_id"] = existing_memory.payload["actor_id"]
         if "role" not in new_metadata and "role" in existing_memory.payload:
             new_metadata["role"] = existing_memory.payload["role"]
diff --git a/tests/memory/test_main.py b/tests/memory/test_main.py
index 9bb655df2..9938baf38 100644
--- a/tests/memory/test_main.py
+++ b/tests/memory/test_main.py
@@ -581,6 +581,52 @@ class TestMetadataNotMutated:
         )
 
 
+def test_update_preserves_actor_id_when_different_actor_updates(mocker):
+    """actor_id must be preserved from the original memory even when the
+    updating caller passes a different actor_id in metadata (issue #4490)."""
+    memory = _build_memory_instance(mocker, Memory)
+    memory.vector_store.get.return_value = MagicMock(
+        payload={
+            "data": "I am player #1",
+            "user_id": "team",
+            "actor_id": "Alice",
+            "created_at": "2026-01-01T00:00:00+00:00",
+        }
+    )
+
+    memory._update_memory(
+        "mem-id", "Player #1 is a good person",
+        {"Player #1 is a good person": [0.1, 0.2, 0.3]},
+        metadata={"user_id": "team", "actor_id": "Bob"},
+    )
+
+    stored = memory.vector_store.update.call_args.kwargs["payload"]
+    assert stored["actor_id"] == "Alice"
+
+
+@pytest.mark.asyncio
+async def test_async_update_preserves_actor_id_when_different_actor_updates(mocker):
+    """Async variant: actor_id must be preserved from the original memory (issue #4490)."""
+    memory = _build_memory_instance(mocker, AsyncMemory)
+    memory.vector_store.get.return_value = MagicMock(
+        payload={
+            "data": "I am player #1",
+            "user_id": "team",
+            "actor_id": "Alice",
+            "created_at": "2026-01-01T00:00:00+00:00",
+        }
+    )
+
+    await memory._update_memory(
+        "mem-id", "Player #1 is a good person",
+        {"Player #1 is a good person": [0.1, 0.2, 0.3]},
+        metadata={"user_id": "team", "actor_id": "Bob"},
+    )
+
+    stored = memory.vector_store.update.call_args.kwargs["payload"]
+    assert stored["actor_id"] == "Alice"
+
+
 def test_normalize_iso_timestamp_to_utc_preserves_naive_values():
     assert _normalize_iso_timestamp_to_utc("2026-03-18T00:00:00") == "2026-03-18T00:00:00"