diff --git a/tests/run-tests.py b/tests/run-tests.py
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -3679,8 +3679,19 @@
     def _use_correct_mercurial(self):
         target_exec = os.path.join(self._custom_bin_dir, b'hg')
         if self._hgcommand != b'hg':
-            # shutil.which only accept bytes from 3.8
-            real_exec = which(self._hgcommand)
+            # _usecorrectpython() may have augmented PATH, so don't used `which`
+            # on the basename of the explicitly provided executable, since that
+            # may end up using another one.
+            #
+            # TODO: figure how why test-run-tests.t fails on Windows if we
+            #  unconditionally join with bindir, and run with --local.  Not
+            #  using --local will run the *.t fine.
+            if self.options.with_hg:
+                real_exec = os.path.join(self._bindir, self._hgcommand)
+            else:
+                # shutil.which only accept bytes from 3.8
+                real_exec = which(self._hgcommand)
+
             if real_exec is None:
                 raise ValueError('could not find exec path for "%s"', real_exec)
             if real_exec == target_exec: