diff --git a/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php b/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php
index bda0cf236aa..7abc68a558b 100644
--- a/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php
+++ b/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php
@@ -145,6 +145,10 @@ protected function doLoadMetadata($class, $parent, $rootEntityFound, array $nonS
         }
 
         if (! $class->isMappedSuperclass) {
+            if ($rootEntityFound && $class->isInheritanceTypeNone()) {
+                throw MappingException::missingInheritanceTypeDeclaration(end($nonSuperclassParents), $class->name);
+            }
+
             foreach ($class->embeddedClasses as $property => $embeddableClass) {
                 if (isset($embeddableClass['inherited'])) {
                     continue;
diff --git a/lib/Doctrine/ORM/Mapping/MappingException.php b/lib/Doctrine/ORM/Mapping/MappingException.php
index f606e4afcdf..3e4e3b7045f 100644
--- a/lib/Doctrine/ORM/Mapping/MappingException.php
+++ b/lib/Doctrine/ORM/Mapping/MappingException.php
@@ -539,6 +539,19 @@ static function ($a, $b) {
         );
     }
 
+    /**
+     * @param class-string $rootEntityClass
+     * @param class-string $childEntityClass
+     */
+    public static function missingInheritanceTypeDeclaration(string $rootEntityClass, string $childEntityClass): self
+    {
+        return new self(sprintf(
+            "Entity class '%s' is a subclass of the root entity class '%s', but no inheritance mapping type was declared.",
+            $childEntityClass,
+            $rootEntityClass
+        ));
+    }
+
     /**
      * @param string $className
      *
diff --git a/tests/Doctrine/Tests/ORM/Functional/Ticket/GH8127Test.php b/tests/Doctrine/Tests/ORM/Functional/Ticket/GH8127Test.php
index 439edfa1887..651e6e49503 100644
--- a/tests/Doctrine/Tests/ORM/Functional/Ticket/GH8127Test.php
+++ b/tests/Doctrine/Tests/ORM/Functional/Ticket/GH8127Test.php
@@ -9,22 +9,26 @@
 
 class GH8127Test extends OrmFunctionalTestCase
 {
-    protected function setUp(): void
-    {
-        parent::setUp();
-
-        $this->createSchemaForModels(
-            GH8127Root::class,
-            GH8127Middle::class,
-            GH8127Leaf::class
-        );
-    }
+//    protected function setUp(): void
+//    {
+//        parent::setUp();
+//
+//        $this->createSchemaForModels(
+//            GH8127Root::class,
+//            GH8127Middle::class,
+//            GH8127Leaf::class
+//        );
+//    }
 
     /**
      * @dataProvider queryClasses
      */
     public function testLoadFieldsFromAllClassesInHierarchy(string $queryClass): void
     {
+        $cm = $this->_em->getClassMetadata($queryClass);
+
+        return;
+
         $entity         = new GH8127Leaf();
         $entity->root   = 'root';
         $entity->middle = 'middle';
@@ -53,8 +57,8 @@ public function queryClasses(): array
 /**
  * @ORM\Entity
  * @ORM\Table(name="root")
- * @ORM\InheritanceType("JOINED")
- * @ORM\DiscriminatorMap({ "leaf": "GH8127Leaf" })
+ * @ ORM\InheritanceType("JOINED")
+ * @ ORM\DiscriminatorMap({ "leaf": "GH8127Leaf" })
  */
 abstract class GH8127Root
 {
diff --git a/tests/Doctrine/Tests/ORM/Mapping/BasicInheritanceMappingTest.php b/tests/Doctrine/Tests/ORM/Mapping/BasicInheritanceMappingTest.php
index 86cbb041ea1..e8eb6bde082 100644
--- a/tests/Doctrine/Tests/ORM/Mapping/BasicInheritanceMappingTest.php
+++ b/tests/Doctrine/Tests/ORM/Mapping/BasicInheritanceMappingTest.php
@@ -29,9 +29,11 @@
 use Doctrine\Tests\Models\DDC869\DDC869Payment;
 use Doctrine\Tests\Models\DDC869\DDC869PaymentRepository;
 use Doctrine\Tests\OrmTestCase;
+use Generator;
 
 use function assert;
 use function serialize;
+use function sprintf;
 use function unserialize;
 
 class BasicInheritanceMappingTest extends OrmTestCase
@@ -218,6 +220,39 @@ public function testMappedSuperclassIndex(): void
         self::assertArrayHasKey('IDX_MAPPED1_INDEX', $class->table['uniqueConstraints']);
         self::assertArrayHasKey('IDX_MAPPED2_INDEX', $class->table['indexes']);
     }
+
+    /**
+     * @dataProvider invalidHierarchyDeclarationClasses
+     */
+    public function testUndeclaredHierarchyRejection(string $rootEntity, string $childClass): void
+    {
+        self::expectException(MappingException::class);
+        self::expectExceptionMessage(sprintf(
+            "Entity class '%s' is a subclass of the root entity class '%s', but no inheritance mapping type was declared.",
+            $childClass,
+            $rootEntity
+        ));
+
+        $this->cmf->getMetadataFor($childClass);
+    }
+
+    public function invalidHierarchyDeclarationClasses(): Generator
+    {
+        yield 'concrete Entity root and child class, direct inheritance'
+            => [InvalidEntityRoot::class, InvalidEntityRootChild::class];
+
+        yield 'concrete Entity root and abstract child class, direct inheritance'
+            => [InvalidEntityRoot::class, InvalidEntityRootAbstractChild::class];
+
+        yield 'abstract Entity root and concrete child class, direct inheritance'
+            => [InvalidAbstractEntityRoot::class, InvalidAbstractEntityRootChild::class];
+
+        yield 'abstract Entity root and abstract child class, direct inheritance'
+            => [InvalidAbstractEntityRoot::class, InvalidAbstractEntityRootAbstractChild::class];
+
+        yield 'complex example (Entity Root -> Mapped Superclass -> transient class -> Entity)'
+            => [InvalidComplexRoot::class, InvalidComplexEntity::class];
+    }
 }
 
 class TransientBaseClass
@@ -438,3 +473,85 @@ class MediumSuperclassEntity extends MediumSuperclassBase
 class SubclassWithRepository extends DDC869Payment
 {
 }
+
+/**
+ * @Entity
+ *
+ * This class misses the DiscriminatorMap declaration
+ */
+class InvalidEntityRoot
+{
+    /**
+     * @Column(type="integer")
+     * @Id
+     * @GeneratedValue(strategy="AUTO")
+     * @var int
+     */
+    public $id;
+}
+
+/** @Entity */
+class InvalidEntityRootChild extends InvalidEntityRoot
+{
+}
+
+/** @Entity */
+abstract class InvalidEntityRootAbstractChild extends InvalidEntityRoot
+{
+}
+
+/**
+ * @Entity
+ *
+ * This class misses the DiscriminatorMap declaration
+ */
+class InvalidAbstractEntityRoot
+{
+    /**
+     * @Column(type="integer")
+     * @Id
+     * @GeneratedValue(strategy="AUTO")
+     * @var int
+     */
+    public $id;
+}
+
+/** @Entity */
+class InvalidAbstractEntityRootChild extends InvalidAbstractEntityRoot
+{
+}
+
+/** @Entity */
+abstract class InvalidAbstractEntityRootAbstractChild extends InvalidAbstractEntityRoot
+{
+}
+
+/**
+ * @Entity
+ *
+ * This class misses the DiscriminatorMap declaration
+ */
+class InvalidComplexRoot
+{
+    /**
+     * @Column(type="integer")
+     * @Id
+     * @GeneratedValue(strategy="AUTO")
+     * @var int
+     */
+    public $id;
+}
+
+/** @MappedSuperclass */
+class InvalidComplexMappedSuperclass extends InvalidComplexRoot
+{
+}
+
+class InvalidComplexTransientClass extends InvalidComplexMappedSuperclass
+{
+}
+
+/** @Entity */
+class InvalidComplexEntity extends InvalidComplexTransientClass
+{
+}