diff --git a/src/Symfony/Component/Serializer/Mapping/Factory/CacheClassMetadataFactory.php b/src/Symfony/Component/Serializer/Mapping/Factory/CacheClassMetadataFactory.php
index 0b904c1440..081917c687 100644
--- a/src/Symfony/Component/Serializer/Mapping/Factory/CacheClassMetadataFactory.php
+++ b/src/Symfony/Component/Serializer/Mapping/Factory/CacheClassMetadataFactory.php
@@ -44,8 +44,7 @@ class CacheClassMetadataFactory implements ClassMetadataFactoryInterface
     public function getMetadataFor($value)
     {
         $class = $this->getClass($value);
-        // Key cannot contain backslashes according to PSR-6
-        $key = strtr($class, '\\', '_');
+        $key = rawurlencode(strtr($class, '\\', '_'));
 
         $item = $this->cacheItemPool->getItem($key);
         if ($item->isHit()) {
diff --git a/src/Symfony/Component/Serializer/Tests/Mapping/Factory/CacheMetadataFactoryTest.php b/src/Symfony/Component/Serializer/Tests/Mapping/Factory/CacheMetadataFactoryTest.php
index 723dc9b049..907311d435 100644
--- a/src/Symfony/Component/Serializer/Tests/Mapping/Factory/CacheMetadataFactoryTest.php
+++ b/src/Symfony/Component/Serializer/Tests/Mapping/Factory/CacheMetadataFactoryTest.php
@@ -63,4 +63,20 @@ class CacheMetadataFactoryTest extends TestCase
 
         $factory->getMetadataFor('Not\Exist');
     }
+
+    public function testAnonymousClass()
+    {
+        $anonymousObject = new class() {
+        };
+
+        $metadata = new ClassMetadata(\get_class($anonymousObject));
+        $decorated = $this->getMockBuilder(ClassMetadataFactoryInterface::class)->getMock();
+        $decorated
+            ->expects($this->once())
+            ->method('getMetadataFor')
+            ->willReturn($metadata);
+
+        $factory = new CacheClassMetadataFactory($decorated, new ArrayAdapter());
+        $this->assertEquals($metadata, $factory->getMetadataFor($anonymousObject));
+    }
 }