diff --git a/UPGRADE.md b/UPGRADE.md
index 09100433fe0..6d40bf48078 100644
--- a/UPGRADE.md
+++ b/UPGRADE.md
@@ -1,5 +1,10 @@
 # Upgrade to 3.1
 
+## Deprecate passing null to `ClassMetadata::fullyQualifiedClassName()`
+
+Passing `null` to `Doctrine\ORM\ClassMetadata::fullyQualifiedClassName()` is
+deprecated and will no longer be possible in 4.0.
+
 ## Deprecate array access
 
 Using array access on instances of the following classes is deprecated:
diff --git a/src/Mapping/ClassMetadata.php b/src/Mapping/ClassMetadata.php
index 9d887be8325..e596537ca49 100644
--- a/src/Mapping/ClassMetadata.php
+++ b/src/Mapping/ClassMetadata.php
@@ -7,6 +7,7 @@
 use BackedEnum;
 use BadMethodCallException;
 use Doctrine\DBAL\Platforms\AbstractPlatform;
+use Doctrine\Deprecations\Deprecation;
 use Doctrine\Instantiator\Instantiator;
 use Doctrine\Instantiator\InstantiatorInterface;
 use Doctrine\ORM\Cache\Exception\NonCacheableEntityAssociation;
@@ -2003,6 +2004,10 @@ protected function _storeAssociationMapping(AssociationMapping $assocMapping): v
      */
     public function setCustomRepositoryClass(string|null $repositoryClassName): void
     {
+        if ($repositoryClassName === null) {
+            $this->customRepositoryClassName = null;
+        }
+
         $this->customRepositoryClassName = $this->fullyQualifiedClassName($repositoryClassName);
     }
 
@@ -2475,6 +2480,13 @@ public function getAssociationMappedByTargetField(string $assocName): string
     public function fullyQualifiedClassName(string|null $className): string|null
     {
         if ($className === null) {
+            Deprecation::trigger(
+                'doctrine/orm',
+                'https://github.com/doctrine/orm/pull/11294',
+                'Passing null to %s is deprecated and will not be supported in Doctrine ORM 4.0',
+                __METHOD__,
+            );
+
             return null;
         }
 
diff --git a/tests/Tests/ORM/Mapping/ClassMetadataTest.php b/tests/Tests/ORM/Mapping/ClassMetadataTest.php
index be44c416aea..acbedcc584c 100644
--- a/tests/Tests/ORM/Mapping/ClassMetadataTest.php
+++ b/tests/Tests/ORM/Mapping/ClassMetadataTest.php
@@ -6,6 +6,7 @@
 
 use ArrayObject;
 use Doctrine\DBAL\Types\Types;
+use Doctrine\Deprecations\PHPUnit\VerifyDeprecations;
 use Doctrine\ORM\Events;
 use Doctrine\ORM\Mapping\ChainTypedFieldMapper;
 use Doctrine\ORM\Mapping\ClassMetadata;
@@ -66,6 +67,8 @@
 
 class ClassMetadataTest extends OrmTestCase
 {
+    use VerifyDeprecations;
+
     public function testClassMetadataInstanceSerialization(): void
     {
         $cm = new ClassMetadata(CmsUser::class);
@@ -1052,6 +1055,15 @@ public function testItAddingLifecycleCallbackOnEmbeddedClassIsIllegal(): void
 
         $metadata->addLifecycleCallback('foo', 'bar');
     }
+
+    public function testGettingAnFQCNForNullIsDeprecated(): void
+    {
+        $metadata = new ClassMetadata(self::class);
+
+        $this->expectDeprecationWithIdentifier('https://github.com/doctrine/orm/pull/11294');
+
+        $metadata->fullyQualifiedClassName(null);
+    }
 }
 
 #[MappedSuperclass]