diff --git a/core/lib/Drupal/Core/Routing/Router.php b/core/lib/Drupal/Core/Routing/Router.php
index 8b649a3ec2a..cbc20c5987b 100644
--- a/core/lib/Drupal/Core/Routing/Router.php
+++ b/core/lib/Drupal/Core/Routing/Router.php
@@ -107,7 +107,19 @@ class Router extends UrlMatcher implements RequestMatcherInterface, RouterInterf
    * {@inheritdoc}
    */
   public function matchRequest(Request $request) {
-    $collection = $this->getInitialRouteCollection($request);
+    try {
+      $collection = $this->getInitialRouteCollection($request);
+    }
+    // PHP 7.4 introduces changes to its serialization format, which mean that
+    // older versions of PHP are unable to unserialize data that is serialized
+    // in PHP 7.4. If the site's version of PHP has been downgraded, then
+    // attempting to unserialize routes from the database will fail, and so the
+    // router needs to be rebuilt on the current PHP version.
+    // See https://www.php.net/manual/en/migration74.incompatible.php.
+    catch (\TypeError $e) {
+      \Drupal::service('router.builder')->rebuild();
+      $collection = $this->getInitialRouteCollection($request);
+    }
     if ($collection->count() === 0) {
       throw new ResourceNotFoundException(sprintf('No routes found for "%s".', $this->currentPath->getPath()));
     }