diff --git a/Lib/multiprocessing/forkserver.py b/Lib/multiprocessing/forkserver.py
index 681af2610e9b37..c91891ff162c2d 100644
--- a/Lib/multiprocessing/forkserver.py
+++ b/Lib/multiprocessing/forkserver.py
@@ -222,6 +222,10 @@ def main(listener_fd, alive_r, preload, main_path=None, sys_path=None,
except ImportError:
pass
+ # gh-135335: flush stdout/stderr in case any of the preloaded modules
+ # wrote to them, otherwise children might inherit buffered data
+ util._flush_std_streams()
+
util._close_stdin()
sig_r, sig_w = os.pipe()
diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
index 75f31d858d3306..a1259ff1d63d18 100644
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -6801,6 +6801,35 @@ def test_child_sys_path(self):
self.assertEqual(child_sys_path[1:], sys.path[1:])
self.assertIsNone(import_error, msg=f"child could not import {self._mod_name}")
+ def test_std_streams_flushed_after_preload(self):
+ # gh-135335: Check fork server flushes standard streams after
+ # preloading modules
+ if multiprocessing.get_start_method() != "forkserver":
+ self.skipTest("forkserver specific test")
+
+ # Create a test module in the temporary directory on the child's path
+ # TODO: This can all be simplified once gh-126631 is fixed and we can
+ # use __main__ instead of a module.
+ dirname = os.path.join(self._temp_dir, 'preloaded_module')
+ init_name = os.path.join(dirname, '__init__.py')
+ os.mkdir(dirname)
+ with open(init_name, "w") as f:
+ cmd = '''if 1:
+ import sys
+ print('stderr', end='', file=sys.stderr)
+ print('stdout', end='', file=sys.stdout)
+ '''
+ f.write(cmd)
+
+ name = os.path.join(os.path.dirname(__file__), 'mp_preload_flush.py')
+ env = {'PYTHONPATH': self._temp_dir}
+ _, out, err = test.support.script_helper.assert_python_ok(name, **env)
+
+ # Check stderr first, as it is more likely to be useful to see in the
+ # event of a failure.
+ self.assertEqual(err.decode().rstrip(), 'stderr')
+ self.assertEqual(out.decode().rstrip(), 'stdout')
+
class MiscTestCase(unittest.TestCase):
def test__all__(self):
diff --git a/Lib/test/mp_preload_flush.py b/Lib/test/mp_preload_flush.py
new file mode 100644
index 00000000000000..3501554d366a21
--- /dev/null
+++ b/Lib/test/mp_preload_flush.py
@@ -0,0 +1,15 @@
+import multiprocessing
+import sys
+
+modname = 'preloaded_module'
+if __name__ == '__main__':
+ if modname in sys.modules:
+ raise AssertionError(f'{modname!r} is not in sys.modules')
+ multiprocessing.set_start_method('forkserver')
+ multiprocessing.set_forkserver_preload([modname])
+ for _ in range(2):
+ p = multiprocessing.Process()
+ p.start()
+ p.join()
+elif modname not in sys.modules:
+ raise AssertionError(f'{modname!r} is not in sys.modules')
diff --git a/Misc/NEWS.d/next/Library/2025-06-10-21-42-04.gh-issue-135335.WnUqb_.rst b/Misc/NEWS.d/next/Library/2025-06-10-21-42-04.gh-issue-135335.WnUqb_.rst
new file mode 100644
index 00000000000000..466ba0d232cd1f
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-06-10-21-42-04.gh-issue-135335.WnUqb_.rst
@@ -0,0 +1,2 @@
+:mod:`multiprocessing`: Flush ``stdout`` and ``stderr`` after preloading
+modules in the ``forkserver``.
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