diff --git a/src/google/adk/artifacts/file_artifact_service.py b/src/google/adk/artifacts/file_artifact_service.py
index b0078e27ce..faf253c97e 100644
--- a/src/google/adk/artifacts/file_artifact_service.py
+++ b/src/google/adk/artifacts/file_artifact_service.py
@@ -62,6 +62,23 @@ def _file_uri_to_path(uri: str) -> Optional[Path]:
   return Path(unquote(parsed.path))
 
 
+def _validate_path_segment(value: str, name: str) -> None:
+  """Validates that a value is safe for use as a single path segment.
+
+  Args:
+    value: The string to validate (e.g. a user_id or session_id).
+    name: A human-readable label used in error messages.
+
+  Raises:
+    InputValidationError: If the value contains path separators or traversal
+      sequences.
+  """
+  if not value:
+    raise InputValidationError(f"{name} must not be empty.")
+  if any(sep in value for sep in ("/", "\\", "..")):
+    raise InputValidationError(f"{name} contains invalid characters: {value!r}")
+
+
 _USER_NAMESPACE_PREFIX = "user:"
 
 
@@ -145,6 +162,7 @@ def _user_artifacts_dir(base_root: Path) -> Path:
 
 def _session_artifacts_dir(base_root: Path, session_id: str) -> Path:
   """Returns the path that stores session-scoped artifacts."""
+  _validate_path_segment(session_id, "session_id")
   return base_root / "sessions" / session_id / "artifacts"
 
 
@@ -220,6 +238,7 @@ def __init__(self, root_dir: Path | str):
 
   def _base_root(self, user_id: str, /) -> Path:
     """Returns the artifacts root directory for a user."""
+    _validate_path_segment(user_id, "user_id")
     return self.root_dir / "users" / user_id
 
   def _scope_root(
diff --git a/tests/unittests/artifacts/test_artifact_service.py b/tests/unittests/artifacts/test_artifact_service.py
index 25294d4909..8c288d58b4 100644
--- a/tests/unittests/artifacts/test_artifact_service.py
+++ b/tests/unittests/artifacts/test_artifact_service.py
@@ -769,6 +769,34 @@ async def test_file_save_artifact_rejects_absolute_path_within_scope(tmp_path):
     )
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    ("user_id", "session_id"),
+    [
+        ("../escape", "sess123"),
+        ("user/../../etc", "sess123"),
+        ("user\\\\..\\\\secret", "sess123"),
+        ("valid_user", "../escape"),
+        ("valid_user", "sess/../../etc"),
+        ("valid_user", "sess\\\\..\\\\secret"),
+    ],
+)
+async def test_file_save_artifact_rejects_path_traversal_in_ids(
+    tmp_path, user_id, session_id
+) -> None:
+  """FileArtifactService rejects user_id/session_id with path traversal."""
+  artifact_service = FileArtifactService(root_dir=tmp_path / "artifacts")
+  part = types.Part(text="content")
+  with pytest.raises(InputValidationError):
+    await artifact_service.save_artifact(
+        app_name="myapp",
+        user_id=user_id,
+        session_id=session_id,
+        filename="safe.txt",
+        artifact=part,
+    )
+
+
 class TestEnsurePart:
   """Tests for the ensure_part normalization helper."""