diff --git a/src/wp-includes/load.php b/src/wp-includes/load.php
index ff68c084104f1..504438cd12c28 100644
--- a/src/wp-includes/load.php
+++ b/src/wp-includes/load.php
@@ -1328,12 +1328,21 @@ function wp_clone( $input_object ) {
  * Determines whether the current request is for the login screen.
  *
  * @since 6.1.0
+ * @since 7.1.0 Checks the default location first
  *
  * @see wp_login_url()
  *
  * @return bool True if inside WordPress login screen, false otherwise.
  */
 function is_login() {
+	// Always check the default location first.
+	if (
+		isset( $_SERVER['SCRIPT_NAME'] )
+		&& str_ends_with( strtolower( $_SERVER['SCRIPT_NAME'] ), '/wp-login.php' )
+	) {
+		return true;
+	}
+
 	return false !== stripos( wp_login_url(), $_SERVER['SCRIPT_NAME'] );
 }
 
diff --git a/tests/phpunit/tests/load/isLogin.php b/tests/phpunit/tests/load/isLogin.php
index 88f8700b97691..e553853fb62ed 100644
--- a/tests/phpunit/tests/load/isLogin.php
+++ b/tests/phpunit/tests/load/isLogin.php
@@ -9,6 +9,42 @@
  */
 class Tests_Load_IsLogin extends WP_UnitTestCase {
 
+	/**
+	 * Original SCRIPT_NAME value, restored after each test.
+	 *
+	 * @var string|null
+	 */
+	private $original_script_name;
+
+	/**
+	 * Original $pagenow value, restored after each test.
+	 *
+	 * @var string|null
+	 */
+	private $original_pagenow;
+
+	public function set_up() {
+		parent::set_up();
+		$this->original_script_name = isset( $_SERVER['SCRIPT_NAME'] ) ? $_SERVER['SCRIPT_NAME'] : null;
+		$this->original_pagenow     = isset( $GLOBALS['pagenow'] ) ? $GLOBALS['pagenow'] : null;
+	}
+
+	public function tear_down() {
+		if ( null === $this->original_script_name ) {
+			unset( $_SERVER['SCRIPT_NAME'] );
+		} else {
+			$_SERVER['SCRIPT_NAME'] = $this->original_script_name;
+		}
+
+		if ( null === $this->original_pagenow ) {
+			unset( $GLOBALS['pagenow'] );
+		} else {
+			$GLOBALS['pagenow'] = $this->original_pagenow;
+		}
+
+		parent::tear_down();
+	}
+
 	/**
 	 * @ticket 19898
 	 */
@@ -19,4 +55,124 @@ public function test_is_login() {
 
 		$this->assertTrue( is_login() );
 	}
+
+	/**
+	 * The default location should be detected regardless of install depth or filename case.
+	 *
+	 * @ticket 65820
+	 *
+	 * @dataProvider data_default_location_script_names
+	 *
+	 * @param string $script_name SCRIPT_NAME to simulate.
+	 */
+	public function test_should_detect_default_location( $script_name ) {
+		$_SERVER['SCRIPT_NAME'] = $script_name;
+
+		$this->assertTrue( is_login() );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_default_location_script_names() {
+		return array(
+			'a root install'         => array( '/wp-login.php' ),
+			'a subdirectory install' => array( '/blog/wp-login.php' ),
+			'an uppercase filename'  => array( '/WP-LOGIN.PHP' ),
+		);
+	}
+
+	/**
+	 * Scripts that merely resemble wp-login.php should not be detected.
+	 *
+	 * @ticket 65820
+	 *
+	 * @dataProvider data_non_login_script_names
+	 *
+	 * @param string $script_name SCRIPT_NAME to simulate.
+	 */
+	public function test_should_not_detect_similar_script_names( $script_name ) {
+		$_SERVER['SCRIPT_NAME'] = $script_name;
+
+		$this->assertFalse( is_login() );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_non_login_script_names() {
+		return array(
+			'the front controller'   => array( '/index.php' ),
+			'a similarly named file' => array( '/my-wp-login.php' ),
+			'a custom entry point without a matching login_url' => array( '/custom-login.php' ),
+		);
+	}
+
+	/**
+	 * Direct execution of wp-login.php is the login screen even when
+	 * the `login_url` filter points somewhere else entirely.
+	 *
+	 * @ticket 65820
+	 */
+	public function test_should_detect_default_location_when_login_url_is_filtered() {
+		add_filter(
+			'login_url',
+			static function () {
+				return 'https://example.org/vip-entrance/';
+			}
+		);
+
+		$_SERVER['SCRIPT_NAME'] = '/wp-login.php';
+
+		$this->assertTrue( is_login() );
+	}
+
+	/**
+	 * Login-relocation plugins (WPS Hide Login, SF Move Login, Rename wp-admin
+	 * login, Admin Login URL Change) serve the login screen through the front
+	 * controller: SCRIPT_NAME stays index.php, `$pagenow` is forced to
+	 * wp-login.php, wp-login.php is `require`d, and `login_url` is filtered to
+	 * the custom slug. is_login() has never detected this context and must
+	 * continue to return false: the default-location check keys on SCRIPT_NAME
+	 * (not `$pagenow`), and the login URL comparison does not match the front
+	 * controller's script name.
+	 *
+	 * @ticket 65820
+	 */
+	public function test_should_not_detect_login_relocated_behind_front_controller() {
+		add_filter(
+			'login_url',
+			static function () {
+				return 'https://example.org/secret-login/';
+			}
+		);
+
+		$_SERVER['SCRIPT_NAME'] = '/index.php';
+		$GLOBALS['pagenow']     = 'wp-login.php';
+
+		$this->assertFalse( is_login() );
+	}
+
+	/**
+	 * A renamed login entry point is not matched by the default-location check
+	 * and must still be detected via the login URL comparison.
+	 *
+	 * @ticket 65820
+	 */
+	public function test_should_fall_through_to_login_url_comparison_for_custom_entry_point() {
+		add_filter(
+			'login_url',
+			static function () {
+				return 'https://example.org/custom-login.php';
+			}
+		);
+
+		$_SERVER['SCRIPT_NAME'] = '/custom-login.php';
+
+		$this->assertTrue( is_login() );
+	}
 }