diff --git a/src/wp-admin/options.php b/src/wp-admin/options.php
index a8c7497ed8..3e2e104214 100644
--- a/src/wp-admin/options.php
+++ b/src/wp-admin/options.php
@@ -279,6 +279,23 @@ if ( 'update' === $action ) { // We are saving settings sent from a settings pag
 			$_POST['gmt_offset']      = $_POST['timezone_string'];
 			$_POST['gmt_offset']      = preg_replace( '/UTC\+?/', '', $_POST['gmt_offset'] );
 			$_POST['timezone_string'] = '';
+		} elseif ( isset( $_POST['timezone_string'] ) && ! in_array( $_POST['timezone_string'], timezone_identifiers_list( DateTimeZone::ALL_WITH_BC ), true ) ) {
+			// Reset to the current value.
+			$current_timezone_string = get_option( 'timezone_string' );
+
+			if ( ! empty( $current_timezone_string ) ) {
+				$_POST['timezone_string'] = $current_timezone_string;
+			} else {
+				$_POST['gmt_offset']      = get_option( 'gmt_offset' );
+				$_POST['timezone_string'] = '';
+			}
+
+			add_settings_error(
+				'general',
+				'settings_updated',
+				__( 'The timezone you have entered is not valid. Please select a valid timezone.' ),
+				'error'
+			);
 		}
 
 		// Handle translation installation.
diff --git a/tests/e2e/specs/general-settings-invalid-timezone.test.js b/tests/e2e/specs/general-settings-invalid-timezone.test.js
new file mode 100644
index 0000000000..9dca6306a4
--- /dev/null
+++ b/tests/e2e/specs/general-settings-invalid-timezone.test.js
@@ -0,0 +1,28 @@
+import { test, expect } from '@wordpress/e2e-test-utils-playwright';
+
+test.describe( 'Settings -> General', () => {
+	const invalidTimezones = [ '', '0', 'Barry/Gary' ];
+
+	for ( const invalidTimezone of invalidTimezones ) {
+		test( `Does not allow saving an invalid timezone string with "${invalidTimezone}"`, async ( { admin, page } ) => {
+			await admin.visitAdminPage( '/options-general.php' );
+
+			// Set the timezone to a valid value.
+			await page.locator( '#timezone_string' ).evaluate( timezone => timezone.value = 'Europe/Lisbon' );
+
+			// Save changes.
+			await page.locator( '#submit' ).click();
+
+			// Set the timezone to an invalid value.
+			await page.locator( '#timezone_string' ).evaluate( ( timezone, invalidTimezone ) => {
+				timezone.options[0].value = invalidTimezone;
+				timezone.value = invalidTimezone;
+			}, invalidTimezone );
+
+			// Save changes.
+			await page.locator( '#submit' ).click();
+
+			await expect( page.locator( '#timezone_string' ) ).toHaveValue( 'Europe/Lisbon' );
+		} );
+	}
+} );