diff --git a/tests/test_runner/test_discover_runner.py b/tests/test_runner/test_discover_runner.py
index 2fc4df31333..327a6625ae1 100644
--- a/tests/test_runner/test_discover_runner.py
+++ b/tests/test_runner/test_discover_runner.py
@@ -1,4 +1,5 @@
 import logging
+import multiprocessing
 import os
 import unittest.loader
 from argparse import ArgumentParser
@@ -36,6 +37,40 @@ def change_loader_patterns(patterns):
         DiscoverRunner.test_loader.testNamePatterns = original_patterns
 
 
+# Isolate from the real environment.
+@mock.patch.dict(os.environ, {}, clear=True)
+@mock.patch.object(multiprocessing, 'cpu_count', return_value=12)
+# Python 3.8 on macOS defaults to 'spawn' mode.
+@mock.patch.object(multiprocessing, 'get_start_method', return_value='fork')
+class DiscoverRunnerParallelArgumentTests(SimpleTestCase):
+    def get_parser(self):
+        parser = ArgumentParser()
+        DiscoverRunner.add_arguments(parser)
+        return parser
+
+    def test_parallel_default(self, *mocked_objects):
+        result = self.get_parser().parse_args([])
+        self.assertEqual(result.parallel, 1)
+
+    def test_parallel_flag(self, *mocked_objects):
+        result = self.get_parser().parse_args(['--parallel'])
+        self.assertEqual(result.parallel, 12)
+
+    def test_parallel_count(self, *mocked_objects):
+        result = self.get_parser().parse_args(['--parallel', '17'])
+        self.assertEqual(result.parallel, 17)
+
+    @mock.patch.dict(os.environ, {'DJANGO_TEST_PROCESSES': 'typo'})
+    def test_parallel_env_var_non_int(self, *mocked_objects):
+        with self.assertRaises(ValueError):
+            self.get_parser().parse_args([])
+
+    def test_parallel_spawn(self, mocked_get_start_method, mocked_cpu_count):
+        mocked_get_start_method.return_value = 'spawn'
+        result = self.get_parser().parse_args(['--parallel'])
+        self.assertEqual(result.parallel, 1)
+
+
 class DiscoverRunnerTests(SimpleTestCase):
 
     @staticmethod