diff --git a/mercurial/utils/procutil.py b/mercurial/utils/procutil.py
--- a/mercurial/utils/procutil.py
+++ b/mercurial/utils/procutil.py
@@ -99,6 +99,18 @@ if isatty(stdout):
     else:
         stdout = os.fdopen(stdout.fileno(), 'wb', 1)
 
+# stderr should be unbuffered
+if pycompat.ispy3:
+    # On Python 3, buffered streams may expose an underlying raw stream. This is
+    # definitively the case for the streams initialized by the interpreter. If
+    # the attribute isn't present, the stream is already unbuffered or doesn't
+    # expose an underlying raw stream, in which case we use the stream as-is.
+    stderr = getattr(stderr, 'raw', stderr)
+elif pycompat.iswindows:
+    # On Windows, stderr is buffered at least when connected to a pipe.
+    stderr = os.fdopen(stderr.fileno(), 'wb', 0)
+# On other platforms, stderr is always unbuffered.
+
 
 findexe = platform.findexe
 _gethgcmd = platform.gethgcmd
diff --git a/tests/test-stdio.py b/tests/test-stdio.py
--- a/tests/test-stdio.py
+++ b/tests/test-stdio.py
@@ -100,6 +100,18 @@ class TestStdio(unittest.TestCase):
             test_stdout_ptys_unbuffered
         )
 
+    def test_stderr_pipes(self):
+        self._test('stderr', _pipes, UNBUFFERED)
+
+    def test_stderr_ptys(self):
+        self._test('stderr', _ptys, UNBUFFERED)
+
+    def test_stderr_pipes_unbuffered(self):
+        self._test('stderr', _pipes, UNBUFFERED, python_args=['-u'])
+
+    def test_stderr_ptys_unbuffered(self):
+        self._test('stderr', _ptys, UNBUFFERED, python_args=['-u'])
+
 
 if __name__ == '__main__':
     import silenttestrunner