diff --git a/tests/phpunit/tests/load/wpIsMaintenanceMode.php b/tests/phpunit/tests/load/wpIsMaintenanceMode.php
new file mode 100644
index 0000000000000..88b9516bbcc6b
--- /dev/null
+++ b/tests/phpunit/tests/load/wpIsMaintenanceMode.php
@@ -0,0 +1,177 @@
+<?php
+
+/**
+ * Tests for wp_is_maintenance_mode().
+ *
+ * @group load
+ *
+ * @covers ::wp_is_maintenance_mode
+ */
+class Tests_Load_WpIsMaintenanceMode extends WP_UnitTestCase {
+
+	/**
+	 * Original $upgrading global.
+	 *
+	 * @var mixed
+	 */
+	private $original_upgrading;
+
+	/**
+	 * Original installing status.
+	 *
+	 * @var bool
+	 */
+	private $original_installing;
+
+	/**
+	 * Original scrape request values.
+	 *
+	 * @var array
+	 */
+	private $original_scrape_request = array();
+
+	public function set_up() {
+		parent::set_up();
+
+		$this->original_upgrading  = $GLOBALS['upgrading'] ?? null;
+		$this->original_installing = wp_installing();
+
+		foreach ( array( 'wp_scrape_key', 'wp_scrape_nonce' ) as $key ) {
+			$this->original_scrape_request[ $key ] = array_key_exists( $key, $_REQUEST ) ? $_REQUEST[ $key ] : null;
+		}
+
+		$this->delete_maintenance_file();
+	}
+
+	public function tear_down() {
+		$this->delete_maintenance_file();
+
+		if ( null !== $this->original_upgrading ) {
+			$GLOBALS['upgrading'] = $this->original_upgrading;
+		} else {
+			unset( $GLOBALS['upgrading'] );
+		}
+
+		wp_installing( $this->original_installing );
+
+		foreach ( $this->original_scrape_request as $key => $value ) {
+			if ( null !== $value ) {
+				$_REQUEST[ $key ] = $value;
+			} else {
+				unset( $_REQUEST[ $key ] );
+			}
+		}
+
+		parent::tear_down();
+	}
+
+	/**
+	 * Writes a `.maintenance` file with the given upgrading timestamp.
+	 *
+	 * Note: wp_is_maintenance_mode() hard-codes the path to ABSPATH . '.maintenance',
+	 * so this writes a real file there rather than into a temporary directory.
+	 * tear_down() removes it. If a test run stops partway through (for example,
+	 * a fatal error or an interrupted process), the file can be left behind, and
+	 * the next WordPress bootstrap will stop in maintenance mode until the
+	 * timestamp expires (up to 10 minutes). If that happens, delete the
+	 * `.maintenance` file from the WordPress root manually.
+	 *
+	 * @param int $upgrading Unix timestamp.
+	 */
+	private function write_maintenance_file( $upgrading ) {
+		file_put_contents( ABSPATH . '.maintenance', '<?php $upgrading = ' . (int) $upgrading . ";\n" );
+	}
+
+	/**
+	 * Deletes the `.maintenance` file if it exists.
+	 */
+	private function delete_maintenance_file() {
+		$file = ABSPATH . '.maintenance';
+
+		if ( file_exists( $file ) ) {
+			unlink( $file );
+		}
+	}
+
+	/**
+	 * @ticket 65819
+	 */
+	public function test_returns_false_when_maintenance_file_is_missing() {
+		$this->assertFalse( wp_is_maintenance_mode() );
+	}
+
+	/**
+	 * @ticket 65819
+	 */
+	public function test_returns_true_when_maintenance_file_is_fresh() {
+		$this->write_maintenance_file( time() );
+
+		$this->assertTrue( wp_is_maintenance_mode() );
+	}
+
+	/**
+	 * @ticket 65819
+	 */
+	public function test_returns_false_when_maintenance_file_is_expired() {
+		$this->write_maintenance_file( time() - ( 10 * MINUTE_IN_SECONDS ) );
+
+		$this->assertFalse( wp_is_maintenance_mode() );
+	}
+
+	/**
+	 * @ticket 65819
+	 */
+	public function test_returns_false_while_installing() {
+		$this->write_maintenance_file( time() );
+		wp_installing( true );
+
+		$this->assertFalse( wp_is_maintenance_mode() );
+	}
+
+	/**
+	 * @ticket 65819
+	 */
+	public function test_returns_false_when_scraping_fatal_errors() {
+		$upgrading = time();
+
+		$this->write_maintenance_file( $upgrading );
+
+		$_REQUEST['wp_scrape_key']   = md5( $upgrading );
+		$_REQUEST['wp_scrape_nonce'] = (string) $upgrading;
+
+		$this->assertFalse( wp_is_maintenance_mode() );
+	}
+
+	/**
+	 * @ticket 65819
+	 */
+	public function test_returns_true_when_scrape_credentials_do_not_match() {
+		$upgrading = time();
+
+		$this->write_maintenance_file( $upgrading );
+
+		$_REQUEST['wp_scrape_key']   = 'invalid';
+		$_REQUEST['wp_scrape_nonce'] = '0';
+
+		$this->assertTrue( wp_is_maintenance_mode() );
+	}
+
+	/**
+	 * @ticket 65819
+	 */
+	public function test_filter_can_disable_maintenance_mode() {
+		$upgrading = time();
+
+		$this->write_maintenance_file( $upgrading );
+
+		$filter = new MockAction();
+		add_filter( 'enable_maintenance_mode', array( $filter, 'filter' ), 10, 2 );
+		add_filter( 'enable_maintenance_mode', '__return_false', 20 );
+
+		$this->assertFalse( wp_is_maintenance_mode() );
+		$this->assertSame(
+			array( array( true, $upgrading ) ),
+			$filter->get_args()
+		);
+	}
+}