diff --git a/contrib/packaging/hgpackaging/py2exe.py b/contrib/packaging/hgpackaging/py2exe.py
--- a/contrib/packaging/hgpackaging/py2exe.py
+++ b/contrib/packaging/hgpackaging/py2exe.py
@@ -118,6 +118,10 @@ def build_py2exe(source_dir: pathlib.Pat
 
     if extra_packages:
         env['HG_PY2EXE_EXTRA_PACKAGES'] = ' '.join(sorted(extra_packages))
+        hgext3rd_extras = sorted(
+            e for e in extra_packages if e.startswith('hgext3rd.'))
+        if hgext3rd_extras:
+            env['HG_PY2EXE_EXTRA_INSTALL_PACKAGES'] = ' '.join(hgext3rd_extras)
     if extra_excludes:
         env['HG_PY2EXE_EXTRA_EXCLUDES'] = ' '.join(sorted(extra_excludes))
     if extra_dll_excludes:
diff --git a/setup.py b/setup.py
--- a/setup.py
+++ b/setup.py
@@ -975,6 +975,12 @@ if sys.version_info[0] == 2:
     packages.extend(['mercurial.thirdparty.concurrent',
                      'mercurial.thirdparty.concurrent.futures'])
 
+if 'HG_PY2EXE_EXTRA_INSTALL_PACKAGES' in os.environ:
+    # py2exe can't cope with namespace packages very well, so we have to
+    # install any hgext3rd.* extensions that we want in the final py2exe
+    # image here. This is gross, but you gotta do what you gotta do.
+    packages.extend(os.environ['HG_PY2EXE_EXTRA_INSTALL_PACKAGES'].split(' '))
+
 common_depends = ['mercurial/bitmanipulation.h',
                   'mercurial/compat.h',
                   'mercurial/cext/util.h']