diff --git a/changelog.d/3609.change.rst b/changelog.d/3609.change.rst
new file mode 100644
index 0000000000..5ed5cc2512
--- /dev/null
+++ b/changelog.d/3609.change.rst
@@ -0,0 +1 @@
+Merge with pypa/distutils@d82d926 including support for DIST_EXTRA_CONFIG in pypa/distutils#177.
diff --git a/docs/deprecated/distutils/configfile.rst b/docs/deprecated/distutils/configfile.rst
index 328936fb40..2a0fbb31d8 100644
--- a/docs/deprecated/distutils/configfile.rst
+++ b/docs/deprecated/distutils/configfile.rst
@@ -36,7 +36,8 @@ consequences:
   :file:`setup.py`
 
 * installers can override anything in :file:`setup.cfg` using the command-line
-  options to :file:`setup.py`
+  options to :file:`setup.py` or by pointing :envvar:`DIST_EXTRA_CONFIG`
+  to another configuration file
 
 The basic syntax of the configuration file is simple:
 
diff --git a/setuptools/_distutils/dist.py b/setuptools/_distutils/dist.py
index 0406ab19cb..1dc25fe541 100644
--- a/setuptools/_distutils/dist.py
+++ b/setuptools/_distutils/dist.py
@@ -7,6 +7,8 @@
 import sys
 import os
 import re
+import pathlib
+import contextlib
 from email import message_from_file
 
 try:
@@ -322,47 +324,40 @@ def find_config_files(self):
         should be parsed.  The filenames returned are guaranteed to exist
         (modulo nasty race conditions).
 
-        There are three possible config files: distutils.cfg in the
-        Distutils installation directory (ie. where the top-level
-        Distutils __inst__.py file lives), a file in the user's home
-        directory named .pydistutils.cfg on Unix and pydistutils.cfg
-        on Windows/Mac; and setup.cfg in the current directory.
-
-        The file in the user's home directory can be disabled with the
-        --no-user-cfg option.
+        There are multiple possible config files:
+        - distutils.cfg in the Distutils installation directory (i.e.
+          where the top-level Distutils __inst__.py file lives)
+        - a file in the user's home directory named .pydistutils.cfg
+          on Unix and pydistutils.cfg on Windows/Mac; may be disabled
+          with the ``--no-user-cfg`` option
+        - setup.cfg in the current directory
+        - a file named by an environment variable
         """
-        files = []
         check_environ()
+        files = [str(path) for path in self._gen_paths() if path.is_file()]
 
-        # Where to look for the system-wide Distutils config file
-        sys_dir = os.path.dirname(sys.modules['distutils'].__file__)
+        if DEBUG:
+            self.announce("using config files: %s" % ', '.join(files))
 
-        # Look for the system config file
-        sys_file = os.path.join(sys_dir, "distutils.cfg")
-        if os.path.isfile(sys_file):
-            files.append(sys_file)
+        return files
 
-        # What to call the per-user config file
-        if os.name == 'posix':
-            user_filename = ".pydistutils.cfg"
-        else:
-            user_filename = "pydistutils.cfg"
+    def _gen_paths(self):
+        # The system-wide Distutils config file
+        sys_dir = pathlib.Path(sys.modules['distutils'].__file__).parent
+        yield sys_dir / "distutils.cfg"
 
-        # And look for the user config file
+        # The per-user config file
+        prefix = '.' * (os.name == 'posix')
+        filename = prefix + 'pydistutils.cfg'
         if self.want_user_cfg:
-            user_file = os.path.join(os.path.expanduser('~'), user_filename)
-            if os.path.isfile(user_file):
-                files.append(user_file)
+            yield pathlib.Path('~').expanduser() / filename
 
         # All platforms support local setup.cfg
-        local_file = "setup.cfg"
-        if os.path.isfile(local_file):
-            files.append(local_file)
+        yield pathlib.Path('setup.cfg')
 
-        if DEBUG:
-            self.announce("using config files: %s" % ', '.join(files))
-
-        return files
+        # Additional config indicated in the environment
+        with contextlib.suppress(TypeError):
+            yield pathlib.Path(os.getenv("DIST_EXTRA_CONFIG"))
 
     def parse_config_files(self, filenames=None):  # noqa: C901
         from configparser import ConfigParser
diff --git a/setuptools/_distutils/tests/test_dist.py b/setuptools/_distutils/tests/test_dist.py
index ddfaf92167..a943832620 100644
--- a/setuptools/_distutils/tests/test_dist.py
+++ b/setuptools/_distutils/tests/test_dist.py
@@ -8,6 +8,7 @@
 import unittest.mock as mock
 
 import pytest
+import jaraco.path
 
 from distutils.dist import Distribution, fix_help_options
 from distutils.cmd import Command
@@ -18,6 +19,9 @@
 from distutils import log
 
 
+pydistutils_cfg = '.' * (os.name == 'posix') + 'pydistutils.cfg'
+
+
 class test_dist(Command):
     """Sample distutils extension command."""
 
@@ -97,26 +101,26 @@ def test_venv_install_options(self, request):
 
         fakepath = '/somedir'
 
-        with open(TESTFN, "w") as f:
-            print(
-                (
-                    "[install]\n"
-                    "install-base = {0}\n"
-                    "install-platbase = {0}\n"
-                    "install-lib = {0}\n"
-                    "install-platlib = {0}\n"
-                    "install-purelib = {0}\n"
-                    "install-headers = {0}\n"
-                    "install-scripts = {0}\n"
-                    "install-data = {0}\n"
-                    "prefix = {0}\n"
-                    "exec-prefix = {0}\n"
-                    "home = {0}\n"
-                    "user = {0}\n"
-                    "root = {0}"
-                ).format(fakepath),
-                file=f,
-            )
+        jaraco.path.build(
+            {
+                TESTFN: f"""
+                    [install]
+                    install-base = {fakepath}
+                    install-platbase = {fakepath}
+                    install-lib = {fakepath}
+                    install-platlib = {fakepath}
+                    install-purelib = {fakepath}
+                    install-headers = {fakepath}
+                    install-scripts = {fakepath}
+                    install-data = {fakepath}
+                    prefix = {fakepath}
+                    exec-prefix = {fakepath}
+                    home = {fakepath}
+                    user = {fakepath}
+                    root = {fakepath}
+                    """,
+            }
+        )
 
         # Base case: Not in a Virtual Environment
         with mock.patch.multiple(sys, prefix='/a', base_prefix='/a'):
@@ -157,12 +161,14 @@ def test_venv_install_options(self, request):
     def test_command_packages_configfile(self, request, clear_argv):
         sys.argv.append("build")
         request.addfinalizer(functools.partial(os.unlink, TESTFN))
-        f = open(TESTFN, "w")
-        try:
-            print("[global]", file=f)
-            print("command_packages = foo.bar, splat", file=f)
-        finally:
-            f.close()
+        jaraco.path.build(
+            {
+                TESTFN: """
+                    [global]
+                    command_packages = foo.bar, splat
+                    """,
+            }
+        )
 
         d = self.create_distribution([TESTFN])
         assert d.get_command_packages() == ["distutils.command", "foo.bar", "splat"]
@@ -240,30 +246,15 @@ def test_announce(self):
         with pytest.raises(ValueError):
             dist.announce(args, kwargs)
 
-    def test_find_config_files_disable(self):
+    def test_find_config_files_disable(self, temp_home):
         # Ticket #1180: Allow user to disable their home config file.
-        temp_home = self.mkdtemp()
-        if os.name == 'posix':
-            user_filename = os.path.join(temp_home, ".pydistutils.cfg")
-        else:
-            user_filename = os.path.join(temp_home, "pydistutils.cfg")
-
-        with open(user_filename, 'w') as f:
-            f.write('[distutils]\n')
-
-        def _expander(path):
-            return temp_home
+        jaraco.path.build({pydistutils_cfg: '[distutils]\n'}, temp_home)
 
-        old_expander = os.path.expanduser
-        os.path.expanduser = _expander
-        try:
-            d = Distribution()
-            all_files = d.find_config_files()
+        d = Distribution()
+        all_files = d.find_config_files()
 
-            d = Distribution(attrs={'script_args': ['--no-user-cfg']})
-            files = d.find_config_files()
-        finally:
-            os.path.expanduser = old_expander
+        d = Distribution(attrs={'script_args': ['--no-user-cfg']})
+        files = d.find_config_files()
 
         # make sure --no-user-cfg disables the user cfg file
         assert len(all_files) - 1 == len(files)
@@ -271,7 +262,7 @@ def _expander(path):
 
 @pytest.mark.usefixtures('save_env')
 @pytest.mark.usefixtures('save_argv')
-class MetadataTestCase(support.TempdirManager):
+class TestMetadata(support.TempdirManager):
     def format_metadata(self, dist):
         sio = io.StringIO()
         dist.metadata.write_pkg_file(sio)
@@ -456,41 +447,20 @@ def test_long_description(self):
         meta = meta.replace('\n' + 8 * ' ', '\n')
         assert long_desc in meta
 
-    def test_custom_pydistutils(self):
-        # fixes #2166
-        # make sure pydistutils.cfg is found
-        if os.name == 'posix':
-            user_filename = ".pydistutils.cfg"
-        else:
-            user_filename = "pydistutils.cfg"
-
-        temp_dir = self.mkdtemp()
-        user_filename = os.path.join(temp_dir, user_filename)
-        f = open(user_filename, 'w')
-        try:
-            f.write('.')
-        finally:
-            f.close()
-
-        try:
-            dist = Distribution()
-
-            # linux-style
-            if sys.platform in ('linux', 'darwin'):
-                os.environ['HOME'] = temp_dir
-                files = dist.find_config_files()
-                assert user_filename in files
-
-            # win32-style
-            if sys.platform == 'win32':
-                # home drive should be found
-                os.environ['USERPROFILE'] = temp_dir
-                files = dist.find_config_files()
-                assert user_filename in files, '{!r} not found in {!r}'.format(
-                    user_filename, files
-                )
-        finally:
-            os.remove(user_filename)
+    def test_custom_pydistutils(self, temp_home):
+        """
+        pydistutils.cfg is found
+        """
+        jaraco.path.build({pydistutils_cfg: ''}, temp_home)
+        config_path = temp_home / pydistutils_cfg
+
+        assert str(config_path) in Distribution().find_config_files()
+
+    def test_extra_pydistutils(self, monkeypatch, tmp_path):
+        jaraco.path.build({'overrides.cfg': ''}, tmp_path)
+        filename = tmp_path / 'overrides.cfg'
+        monkeypatch.setenv('DIST_EXTRA_CONFIG', str(filename))
+        assert str(filename) in Distribution().find_config_files()
 
     def test_fix_help_options(self):
         help_tuples = [('a', 'b', 'c', 'd'), (1, 2, 3, 4)]
@@ -498,9 +468,10 @@ def test_fix_help_options(self):
         assert fancy_options[0] == ('a', 'b', 'c')
         assert fancy_options[1] == (1, 2, 3)
 
-    def test_show_help(self):
+    def test_show_help(self, request):
         # smoke test, just makes sure some help is displayed
-        self.addCleanup(log.set_threshold, log._global_log.threshold)
+        reset_log = functools.partial(log.set_threshold, log._global_log.threshold)
+        request.addfinalizer(reset_log)
         dist = Distribution()
         sys.argv = []
         dist.help = 1
diff --git a/setuptools/_distutils/tests/test_util.py b/setuptools/_distutils/tests/test_util.py
index 605b0d40b7..070a277069 100644
--- a/setuptools/_distutils/tests/test_util.py
+++ b/setuptools/_distutils/tests/test_util.py
@@ -136,17 +136,16 @@ def _join(*path):
         # XXX platforms to be covered: mac
 
     def test_check_environ(self):
-        util._environ_checked = 0
+        util.check_environ.cache_clear()
         os.environ.pop('HOME', None)
 
         check_environ()
 
         assert os.environ['PLAT'] == get_platform()
-        assert util._environ_checked == 1
 
     @pytest.mark.skipif("os.name != 'posix'")
     def test_check_environ_getpwuid(self):
-        util._environ_checked = 0
+        util.check_environ.cache_clear()
         os.environ.pop('HOME', None)
 
         import pwd
@@ -159,7 +158,7 @@ def test_check_environ_getpwuid(self):
             check_environ()
             assert os.environ['HOME'] == '/home/distutils'
 
-        util._environ_checked = 0
+        util.check_environ.cache_clear()
         os.environ.pop('HOME', None)
 
         # bpo-10496: Catch pwd.getpwuid() error
diff --git a/setuptools/_distutils/util.py b/setuptools/_distutils/util.py
index d95992ec99..4763202b67 100644
--- a/setuptools/_distutils/util.py
+++ b/setuptools/_distutils/util.py
@@ -11,6 +11,8 @@
 import subprocess
 import sys
 import sysconfig
+import functools
+
 from distutils.errors import DistutilsPlatformError, DistutilsByteCompileError
 from distutils.dep_util import newer
 from distutils.spawn import spawn
@@ -170,9 +172,7 @@ def change_root(new_root, pathname):
     raise DistutilsPlatformError(f"nothing known about platform '{os.name}'")
 
 
-_environ_checked = 0
-
-
+@functools.lru_cache()
 def check_environ():
     """Ensure that 'os.environ' has all the environment variables we
     guarantee that users can use in config files, command-line options,
@@ -181,10 +181,6 @@ def check_environ():
       PLAT - description of the current platform, including hardware
              and OS (see 'get_platform()')
     """
-    global _environ_checked
-    if _environ_checked:
-        return
-
     if os.name == 'posix' and 'HOME' not in os.environ:
         try:
             import pwd
@@ -198,8 +194,6 @@ def check_environ():
     if 'PLAT' not in os.environ:
         os.environ['PLAT'] = get_platform()
 
-    _environ_checked = 1
-
 
 def subst_vars(s, local_vars):
     """