diff --git a/src/wp-includes/formatting.php b/src/wp-includes/formatting.php
index 8d41efffdc..fbb9633d2a 100644
--- a/src/wp-includes/formatting.php
+++ b/src/wp-includes/formatting.php
@@ -4923,7 +4923,7 @@ function sanitize_option( $option, $value ) {
 			break;
 
 		case 'timezone_string':
-			$allowed_zones = timezone_identifiers_list();
+			$allowed_zones = timezone_identifiers_list( DateTimeZone::ALL_WITH_BC );
 			if ( ! in_array( $value, $allowed_zones, true ) && ! empty( $value ) ) {
 				$error = __( 'The timezone you have entered is not valid. Please select a valid timezone.' );
 			}
diff --git a/tests/phpunit/tests/option/sanitizeOption.php b/tests/phpunit/tests/option/sanitizeOption.php
index bc0dd0843b..dec3cf7ced 100644
--- a/tests/phpunit/tests/option/sanitizeOption.php
+++ b/tests/phpunit/tests/option/sanitizeOption.php
@@ -69,6 +69,12 @@ class Tests_Option_SanitizeOption extends WP_UnitTestCase {
 			array( 'timezone_string', 0, 0 ),
 			array( 'timezone_string', 'Europe/London', 'Europe/London' ),
 			array( 'timezone_string', get_option( 'timezone_string' ), 'invalid' ),
+			// @ticket 56468
+			'deprecated timezone string is accepted as valid' => array(
+				'option_name' => 'timezone_string',
+				'sanitized'   => 'America/Buenos_Aires',
+				'original'    => 'America/Buenos_Aires',
+			),
 			array( 'permalink_structure', '', '' ),
 			array( 'permalink_structure', '/%year%/%20%postname%', '/%year%/ %postname%' ),
 			array( 'default_role', 'subscriber', 'subscriber' ),