diff --git a/src/google/adk/artifacts/gcs_artifact_service.py b/src/google/adk/artifacts/gcs_artifact_service.py
index f8706dedbd..9f4e7adced 100644
--- a/src/google/adk/artifacts/gcs_artifact_service.py
+++ b/src/google/adk/artifacts/gcs_artifact_service.py
@@ -32,6 +32,7 @@
 from google.genai import types
 from typing_extensions import override
 
+from . import artifact_util
 from ..errors.input_validation_error import InputValidationError
 from .base_artifact_service import ArtifactVersion
 from .base_artifact_service import BaseArtifactService
@@ -230,9 +231,21 @@ def _save_artifact(
           content_type="text/plain",
       )
     elif artifact.file_data:
-      raise NotImplementedError(
-          "Saving artifact with file_data is not supported yet in"
-          " GcsArtifactService."
+      if not artifact.file_data.file_uri:
+        raise InputValidationError("Artifact file_data must have a file_uri.")
+      if artifact_util.is_artifact_ref(artifact):
+        if not artifact_util.parse_artifact_uri(artifact.file_data.file_uri):
+          raise InputValidationError(
+              f"Invalid artifact reference URI: {artifact.file_data.file_uri}"
+          )
+      # Store the URI as blob metadata; no content to upload.
+      blob.metadata = {
+          **(blob.metadata or {}),
+          "file_uri": artifact.file_data.file_uri,
+      }
+      blob.upload_from_string(
+          b"",
+          content_type=artifact.file_data.mime_type or None,
       )
     else:
       raise InputValidationError(
@@ -263,15 +276,25 @@ def _load_artifact(
     blob_name = self._get_blob_name(
         app_name, user_id, filename, version, session_id
     )
-    blob = self.bucket.blob(blob_name)
+    blob = self.bucket.get_blob(blob_name)
+    if blob is None:
+      return None
+
+    # If the artifact was saved as a file_data URI reference, restore it.
+    if blob.metadata and "file_uri" in blob.metadata:
+      return types.Part(
+          file_data=types.FileData(
+              file_uri=blob.metadata["file_uri"],
+              mime_type=blob.content_type or None,
+          )
+      )
 
     artifact_bytes = blob.download_as_bytes()
     if not artifact_bytes:
       return None
-    artifact = types.Part.from_bytes(
+    return types.Part.from_bytes(
         data=artifact_bytes, mime_type=blob.content_type
     )
-    return artifact
 
   def _list_artifact_keys(
       self, app_name: str, user_id: str, session_id: Optional[str]
diff --git a/tests/unittests/artifacts/test_artifact_service.py b/tests/unittests/artifacts/test_artifact_service.py
index 8b82397097..7a0ef8879c 100644
--- a/tests/unittests/artifacts/test_artifact_service.py
+++ b/tests/unittests/artifacts/test_artifact_service.py
@@ -865,6 +865,119 @@ def test_converts_text_dict(self):
     assert result.text == "hello world"
 
 
+# ---------------------------------------------------------------------------
+# GCS file_data (URI reference) tests
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_gcs_save_artifact_with_external_gcs_uri():
+  """GcsArtifactService saves and loads a gs:// file_data URI reference."""
+  service = mock_gcs_artifact_service()
+  artifact = types.Part(
+      file_data=types.FileData(
+          file_uri="gs://my-bucket/report.pdf",
+          mime_type="application/pdf",
+      )
+  )
+
+  version = await service.save_artifact(
+      app_name="app",
+      user_id="user1",
+      session_id="sess1",
+      filename="report.pdf",
+      artifact=artifact,
+  )
+  assert version == 0
+
+  loaded = await service.load_artifact(
+      app_name="app",
+      user_id="user1",
+      session_id="sess1",
+      filename="report.pdf",
+  )
+  assert loaded is not None
+  assert loaded.file_data is not None
+  assert loaded.file_data.file_uri == "gs://my-bucket/report.pdf"
+  assert loaded.file_data.mime_type == "application/pdf"
+
+
+@pytest.mark.asyncio
+async def test_gcs_save_artifact_with_artifact_ref_uri():
+  """GcsArtifactService saves and loads an internal artifact:// URI reference."""
+  service = mock_gcs_artifact_service()
+  artifact_ref_uri = "artifact://apps/app/users/user1/sessions/sess1/artifacts/source.txt/versions/0"
+  artifact = types.Part(
+      file_data=types.FileData(
+          file_uri=artifact_ref_uri,
+          mime_type="text/plain",
+      )
+  )
+
+  version = await service.save_artifact(
+      app_name="app",
+      user_id="user1",
+      session_id="sess1",
+      filename="ref.txt",
+      artifact=artifact,
+  )
+  assert version == 0
+
+  loaded = await service.load_artifact(
+      app_name="app",
+      user_id="user1",
+      session_id="sess1",
+      filename="ref.txt",
+  )
+  assert loaded is not None
+  assert loaded.file_data is not None
+  assert loaded.file_data.file_uri == artifact_ref_uri
+
+
+@pytest.mark.asyncio
+async def test_gcs_save_artifact_file_data_without_mime_type():
+  """GcsArtifactService handles file_data with no mime_type."""
+  service = mock_gcs_artifact_service()
+  artifact = types.Part(
+      file_data=types.FileData(file_uri="gs://my-bucket/data.bin")
+  )
+
+  version = await service.save_artifact(
+      app_name="app",
+      user_id="user1",
+      session_id="sess1",
+      filename="data.bin",
+      artifact=artifact,
+  )
+  assert version == 0
+
+  loaded = await service.load_artifact(
+      app_name="app",
+      user_id="user1",
+      session_id="sess1",
+      filename="data.bin",
+  )
+  assert loaded is not None
+  assert loaded.file_data is not None
+  assert loaded.file_data.file_uri == "gs://my-bucket/data.bin"
+
+
+@pytest.mark.asyncio
+async def test_gcs_save_artifact_file_data_missing_uri_raises():
+  """GcsArtifactService raises InputValidationError when file_uri is empty."""
+  service = mock_gcs_artifact_service()
+  artifact = types.Part(file_data=types.FileData(file_uri=""))
+
+  with pytest.raises(InputValidationError):
+    await service.save_artifact(
+        app_name="app",
+        user_id="user1",
+        session_id="sess1",
+        filename="empty.bin",
+        artifact=artifact,
+    )
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize(
     "service_type",