diff --git a/docs/versionhistory.rst b/docs/versionhistory.rst
index 2660524b..a571bfd1 100644
--- a/docs/versionhistory.rst
+++ b/docs/versionhistory.rst
@@ -3,6 +3,11 @@ Version history
 
 This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 
+**UNRELEASED**
+
+- Fixed ``RuntimeError`` on asyncio when a ``CancelledError`` is raised from a task spawned through
+  a ``BlockingPortal`` (`#357 <https://github.com/agronholm/anyio/issues/357>`_)
+
 **3.3.0**
 
 - Added asynchronous ``Path`` class
diff --git a/src/anyio/from_thread.py b/src/anyio/from_thread.py
index 26d4a262..d845f993 100644
--- a/src/anyio/from_thread.py
+++ b/src/anyio/from_thread.py
@@ -162,7 +162,7 @@ async def stop(self, cancel_remaining: bool = False) -> None:
     async def _call_func(self, func: Callable, args: tuple, kwargs: Dict[str, Any],
                          future: Future) -> None:
         def callback(f: Future) -> None:
-            if f.cancelled():
+            if f.cancelled() and self._event_loop_thread_id not in (None, threading.get_ident()):
                 self.call(scope.cancel)
 
         try:
diff --git a/tests/test_from_thread.py b/tests/test_from_thread.py
index 5339ea88..63fbc36c 100644
--- a/tests/test_from_thread.py
+++ b/tests/test_from_thread.py
@@ -376,3 +376,14 @@ def taskfunc(*, task_status: TaskStatus) -> None:
             future, start_value = portal.start_task(
                 taskfunc, name='testname')  # type: ignore[arg-type]
             assert start_value == 'testname'
+
+    @pytest.mark.parametrize('anyio_backend', ['asyncio'])
+    async def test_asyncio_run_sync_called(self, caplog):
+        """Regression test for #357."""
+        async def in_loop():
+            raise CancelledError
+
+        async with BlockingPortal() as portal:
+            await to_thread.run_sync(portal.start_task_soon, in_loop)
+
+        assert not caplog.text