diff --git a/docs/changelog/1313.bugfix.rst b/docs/changelog/1313.bugfix.rst
new file mode 100644
index 000000000..8b89489dd
--- /dev/null
+++ b/docs/changelog/1313.bugfix.rst
@@ -0,0 +1 @@
+Ensure ``TOX_WORK_DIR`` is a native string in ``os.environ`` - by :user:`asottile`.
diff --git a/src/tox/session/__init__.py b/src/tox/session/__init__.py
index e93886841..048feb94e 100644
--- a/src/tox/session/__init__.py
+++ b/src/tox/session/__init__.py
@@ -63,7 +63,7 @@ def main(args):
     try:
         config = load_config(args)
         config.logdir.ensure(dir=1)
-        with set_os_env_var("TOX_WORK_DIR", config.toxworkdir):
+        with set_os_env_var(str("TOX_WORK_DIR"), config.toxworkdir):
             session = build_session(config)
             exit_code = session.runcommand()
         if exit_code is None:
diff --git a/tests/unit/session/test_session.py b/tests/unit/session/test_session.py
index b90430395..df5814bb6 100644
--- a/tests/unit/session/test_session.py
+++ b/tests/unit/session/test_session.py
@@ -285,6 +285,10 @@ def assert_popen_env(res):
         if tox_id != "GLOB":
             assert env["TOX_ENV_NAME"] == tox_id
             assert env["TOX_ENV_DIR"] == os.path.join(res.cwd, ".tox", tox_id)
+        # ensure native strings for environ for windows
+        for k, v in env.items():
+            assert type(k) is str, (k, v, type(k))
+            assert type(v) is str, (k, v, type(v))
 
 
 def test_command_prev_post_ok(cmd, initproj, mock_venv):