diff --git a/system/Test/CIUnitTestCase.php b/system/Test/CIUnitTestCase.php
index d86e543a9a61..05e4c647f0ed 100644
--- a/system/Test/CIUnitTestCase.php
+++ b/system/Test/CIUnitTestCase.php
@@ -273,6 +273,8 @@ protected function tearDown(): void
 
         // Check for other trait methods
         $this->callTraitMethods('tearDown');
+
+        $this->resetIsWindowsMock();
     }
 
     /**
@@ -318,6 +320,14 @@ protected function resetServices(bool $initAutoloader = true)
         Services::reset($initAutoloader);
     }
 
+    /**
+     * Resets the mocked is_windows() function back to default state.
+     */
+    protected function resetIsWindowsMock(): void
+    {
+        is_windows(null);
+    }
+
     /**
      * Injects the mock Cache driver to prevent filesystem collisions.
      *
diff --git a/tests/system/CommonFunctionsTest.php b/tests/system/CommonFunctionsTest.php
index b4901a5e8d93..f1ce25c04aa1 100644
--- a/tests/system/CommonFunctionsTest.php
+++ b/tests/system/CommonFunctionsTest.php
@@ -863,7 +863,7 @@ public function testIsWindowsUsingMock(): void
         $this->assertFalse(is_windows());
         $this->assertNotTrue(is_windows());
 
-        is_windows();
+        is_windows(null);
         $this->assertSame(str_contains(php_uname(), 'Windows'), is_windows());
         $this->assertSame(defined('PHP_WINDOWS_VERSION_MAJOR'), is_windows());
     }
diff --git a/tests/system/Test/TestCaseTest.php b/tests/system/Test/TestCaseTest.php
index 6905d825533a..611c3fe0ebd4 100644
--- a/tests/system/Test/TestCaseTest.php
+++ b/tests/system/Test/TestCaseTest.php
@@ -93,4 +93,29 @@ public function testCloseEnoughStringBadLength(): void
         $result = $this->assertCloseEnoughString('apples & oranges', 'apples');
         $this->assertFalse($result, 'Different string lengths should have returned false');
     }
+
+    public function testTearDownResetsIsWindowsMock(): void
+    {
+        $testCase = new class ('test') extends CIUnitTestCase {
+            protected $tearDownMethods = ['customTearDown'];
+
+            protected function customTearDown(): void
+            {
+            }
+
+            public function triggerTearDown(): void
+            {
+                $this->tearDown();
+            }
+        };
+
+        $default = DIRECTORY_SEPARATOR === '\\';
+
+        is_windows(! $default);
+        $this->assertSame(! $default, is_windows());
+
+        $testCase->triggerTearDown();
+
+        $this->assertSame($default, is_windows());
+    }
 }