--- a/Loader/Configurator/Traits/PrefixTrait.php
+++ b/Loader/Configurator/Traits/PrefixTrait.php
@@ -39,6 +39,9 @@
                         $localizedRoute->setPath($localePrefix.(!$trailingSlashOnRoot && '/' === $route->getPath() ? '' : $route->getPath()));
                         $routes->add($name.'.'.$locale, $localizedRoute, $priority);
                     }
+                } elseif (false === $locale) {
+                    // Explicitly non-prefixed non-localized route.
+                    $route->setDefault('_locale', null);
                 } elseif (!isset($prefix[$locale])) {
                     throw new \InvalidArgumentException(\sprintf('Route "%s" with locale "%s" is missing a corresponding prefix in its parent collection.', $name, $locale));
                 } else {
--- a/Attribute/Route.php
+++ b/Attribute/Route.php
@@ -51,7 +51,7 @@
         array|string $schemes = [],
         private ?string $condition = null,
         private ?int $priority = null,
-        ?string $locale = null,
+        string|false|null $locale = null,
         ?string $format = null,
         ?bool $utf8 = null,
         ?bool $stateless = null,