diff --git a/python/langsmith/client.py b/python/langsmith/client.py
index 1647b790d..9dd1442a1 100644
--- a/python/langsmith/client.py
+++ b/python/langsmith/client.py
@@ -1620,6 +1620,9 @@ def update_run(
         events: Optional[Sequence[dict]] = None,
         extra: Optional[Dict] = None,
         tags: Optional[List[str]] = None,
+        attachments: Optional[
+            Dict[str, tuple[str, bytes] | ls_schemas.Attachment]
+        ] = None,
         **kwargs: Any,
     ) -> None:
         """Update a run in the LangSmith API.
@@ -1644,6 +1647,9 @@ def update_run(
             The extra information for the run.
         tags : List[str] or None, default=None
             The tags for the run.
+        attachments: dict[str, ls_schemas.Attachment] or None, default=None
+            A dictionary of attachments to add to the run. The keys are the attachment names,
+            and the values are Attachment objects containing the data and mime type.
         **kwargs : Any
             Kwargs are ignored.
         """
@@ -1658,6 +1664,8 @@ def update_run(
             "session_id": kwargs.pop("session_id", None),
             "session_name": kwargs.pop("session_name", None),
         }
+        if attachments:
+            data["attachments"] = attachments
         use_multipart = (
             self.tracing_queue is not None
             # batch ingest requires trace_id and dotted_order to be set
diff --git a/python/langsmith/run_trees.py b/python/langsmith/run_trees.py
index d3083e4f6..63f0cb4e5 100644
--- a/python/langsmith/run_trees.py
+++ b/python/langsmith/run_trees.py
@@ -301,6 +301,15 @@ def post(self, exclude_child_runs: bool = True) -> None:
         """Post the run tree to the API asynchronously."""
         kwargs = self._get_dicts_safe()
         self.client.create_run(**kwargs)
+        if attachments := kwargs.get("attachments"):
+            keys = [str(name) for name in attachments]
+            self.events.append(
+                {
+                    "name": "uploaded_attachment",
+                    "time": datetime.now(timezone.utc).isoformat(),
+                    "message": set(keys),
+                }
+            )
         if not exclude_child_runs:
             for child_run in self.child_runs:
                 child_run.post(exclude_child_runs=False)
@@ -309,6 +318,26 @@ def patch(self) -> None:
         """Patch the run tree to the API in a background thread."""
         if not self.end_time:
             self.end()
+        attachments = self.attachments
+        try:
+            # Avoid loading the same attachment twice
+            if attachments:
+                uploaded = next(
+                    (
+                        ev
+                        for ev in self.events
+                        if ev.get("name") == "uploaded_attachment"
+                    ),
+                    None,
+                )
+                if uploaded:
+                    attachments = {
+                        a: v
+                        for a, v in attachments.items()
+                        if a not in uploaded["message"]
+                    }
+        except Exception as e:
+            logger.warning(f"Error filtering attachments to upload: {e}")
         self.client.update_run(
             name=self.name,
             run_id=self.id,
@@ -322,6 +351,7 @@ def patch(self) -> None:
             events=self.events,
             tags=self.tags,
             extra=self.extra,
+            attachments=attachments,
         )
 
     def wait(self) -> None:
diff --git a/python/tests/unit_tests/test_run_helpers.py b/python/tests/unit_tests/test_run_helpers.py
index dbbbe1adf..34df400e7 100644
--- a/python/tests/unit_tests/test_run_helpers.py
+++ b/python/tests/unit_tests/test_run_helpers.py
@@ -1714,7 +1714,11 @@ def my_func(
             val: int,
             att1: ls_schemas.Attachment,
             att2: Annotated[tuple, ls_schemas.Attachment],
+            run_tree: RunTree,
         ):
+            run_tree.attachments["anoutput"] = ls_schemas.Attachment(
+                mime_type="text/plain", data=b"noidea"
+            )
             return "foo"
 
         mock_client = _get_mock_client(
@@ -1739,11 +1743,15 @@ def my_func(
             )
             assert result == "foo"
 
-        calls = _get_calls(mock_client)
-        datas = _get_multipart_data(calls)
+        for _ in range(10):
+            calls = _get_calls(mock_client)
+            datas = _get_multipart_data(calls)
+            if len(datas) >= 7:
+                break
+            time.sleep(1)
 
-        # main run, inputs, outputs, events, att1, att2
-        assert len(datas) == 6
+        # main run, inputs, outputs, events, att1, att2, anoutput
+        assert len(datas) == 7
         # First 4 are type application/json (run, inputs, outputs, events)
         trace_id = datas[0][0].split(".")[1]
         _, (_, run_stuff) = next(
@@ -1760,7 +1768,7 @@ def my_func(
             data for data in datas if data[0] == f"post.{trace_id}.inputs"
         )
         assert json.loads(inputs) == {"val": 42}
-        # last two are the mime types provided
+        # last three are the mime types provided
         _, (mime_type1, content1) = next(
             data for data in datas if data[0] == f"attachment.{trace_id}.att1"
         )
@@ -1772,3 +1780,10 @@ def my_func(
         )
         assert mime_type2 == "application/octet-stream"
         assert content2 == b"content2"
+
+        # Assert that anoutput is uploaded
+        _, (mime_type_output, content_output) = next(
+            data for data in datas if data[0] == f"attachment.{trace_id}.anoutput"
+        )
+        assert mime_type_output == "text/plain"
+        assert content_output == b"noidea"