diff --git a/tests/conftest.py b/tests/conftest.py
index 3a8ae69e..486d4b0a 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -6,16 +6,52 @@
     :copyright: (c) 2015 by the Flask Team, see AUTHORS for more details.
     :license: BSD, see LICENSE for more details.
 """
-import flask
 import gc
 import os
-import sys
 import pkgutil
-import pytest
+import sys
 import textwrap
+
+import pytest
+from _pytest import monkeypatch
+
+import flask
 from flask import Flask as _Flask
 
 
+@pytest.fixture(scope='session', autouse=True)
+def _standard_os_environ():
+    """Set up ``os.environ`` at the start of the test session to have
+    standard values. Returns a list of operations that is used by
+    :func:`._reset_os_environ` after each test.
+    """
+    mp = monkeypatch.MonkeyPatch()
+    out = (
+        (os.environ, 'FLASK_APP', monkeypatch.notset),
+        (os.environ, 'FLASK_ENV', monkeypatch.notset),
+        (os.environ, 'FLASK_DEBUG', monkeypatch.notset),
+        (os.environ, 'FLASK_RUN_FROM_CLI', monkeypatch.notset),
+        (os.environ, 'WERKZEUG_RUN_MAIN', monkeypatch.notset),
+    )
+
+    for _, key, value in out:
+        if value is monkeypatch.notset:
+            mp.delenv(key, False)
+        else:
+            mp.setenv(key, value)
+
+    yield out
+    mp.undo()
+
+
+@pytest.fixture(autouse=True)
+def _reset_os_environ(monkeypatch, _standard_os_environ):
+    """Reset ``os.environ`` to the standard environ after each test,
+    in case a test changed something without cleaning up.
+    """
+    monkeypatch._setitem.extend(_standard_os_environ)
+
+
 class Flask(_Flask):
     testing = True
     secret_key = 'test key'
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 811ef0c8..d9216f3d 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -23,9 +23,11 @@ from _pytest.monkeypatch import notset
 from click.testing import CliRunner
 
 from flask import Flask, current_app
-from flask.cli import AppGroup, FlaskGroup, NoAppException, ScriptInfo, dotenv, \
-    find_best_app, get_version, load_dotenv, locate_app, prepare_import, \
+from flask.cli import (
+    AppGroup, FlaskGroup, NoAppException, ScriptInfo, dotenv,
+    find_best_app, get_version, load_dotenv, locate_app, prepare_import,
     with_appcontext
+)
 
 cwd = os.getcwd()
 test_path = os.path.abspath(os.path.join(
@@ -33,19 +35,6 @@ test_path = os.path.abspath(os.path.join(
 ))
 
 
-@pytest.fixture(autouse=True)
-def manage_os_environ(monkeypatch):
-    # can't use monkeypatch.delitem since we don't want to restore a value
-    os.environ.pop('FLASK_APP', None)
-    os.environ.pop('FLASK_DEBUG', None)
-    # use monkeypatch internals to force-delete environ keys
-    monkeypatch._setitem.extend((
-        (os.environ, 'FLASK_APP', notset),
-        (os.environ, 'FLASK_DEBUG', notset),
-        (os.environ, 'FLASK_RUN_FROM_CLI', notset),
-    ))
-
-
 @pytest.fixture
 def runner():
     return CliRunner()