diff --git a/core/lib/Drupal/Core/TypedData/TypedDataManager.php b/core/lib/Drupal/Core/TypedData/TypedDataManager.php
index 273066d411..228722fdf0 100644
--- a/core/lib/Drupal/Core/TypedData/TypedDataManager.php
+++ b/core/lib/Drupal/Core/TypedData/TypedDataManager.php
@@ -169,10 +169,6 @@ public function getPropertyInstance(TypedDataInterface $object, $property_name,
       $parts[] = json_encode($settings);
     }
     // Property path for the requested data object.
-    // the original code is:
-    // code$parts[] = $object->getPropertyPath() . '.' . (is_numeric($property_name) ? 0 : $property_name);
-    // When creating a list item , use 0 in the key as all items look the same.
-    // But wen Map data use setPropertyDefinition, We may pass an array that has numerical key with different type of value
     $parts[] = $object->getPropertyPath() . '.' . $property_name;
     $key = implode(':', $parts);
 
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityTest/EntityTestMapItemNormalizerTest.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityTest/EntityTestMapItemNormalizerTest.php
new file mode 100644
index 0000000000..018706ee61
--- /dev/null
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityTest/EntityTestMapItemNormalizerTest.php
@@ -0,0 +1,95 @@
+<?php
+
+namespace Drupal\Tests\rest\Functional\EntityResource\EntityTest;
+
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+use Drupal\Tests\rest\Functional\AnonResourceTestTrait;
+
+/**
+ * Test that MapItem are correctly exposed in REST.
+ *
+ * @group rest
+ */
+class EntityTestMapItemNormalizerTest extends EntityTestResourceTestBase {
+
+  use AnonResourceTestTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'json';
+
+  protected static $mapValue = [
+    'key1' => 'value',
+    'key2' => 'no, val you',
+    'nested' => [
+      'bird' => 'robin',
+      'doll' => 'Russian',
+    ],
+  ];
+
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'application/json';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getExpectedNormalizedEntity() {
+    $expected = parent::getExpectedNormalizedEntity();
+    // The 'non_exposed_value' property in test field type will not return in
+    // normalization because setExposed(TRUE) was not called for this property.
+    // @see \Drupal\entity_test\Plugin\Field\FieldType\ExposedPropertyTestFieldItem::propertyDefinitions
+    $expected['field_map'] = [
+      [
+        'freeform' => static::$mapValue,
+      ],
+    ];
+    return $expected;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function createEntity() {
+    if (!FieldStorageConfig::loadByName('entity_test', 'field_map')) {
+      FieldStorageConfig::create([
+        'entity_type' => 'entity_test',
+        'field_name' => 'field_map',
+        'type' => 'map_test',
+        'cardinality' => 1,
+        'translatable' => FALSE,
+      ])->save();
+      FieldConfig::create([
+        'entity_type' => 'entity_test',
+        'field_name' => 'field_map',
+        'bundle' => 'entity_test',
+        'label' => 'Test field with map property',
+      ])->save();
+    }
+
+    $entity = parent::createEntity();
+    $entity->field_map = [
+      'freeform' => static::$mapValue,
+    ];
+    $entity->save();
+    return $entity;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getNormalizedPostEntity() {
+    return parent::getNormalizedPostEntity() + [
+      'field_map' => [
+        [
+          'freeform' => static::$mapValue,
+        ],
+      ],
+    ];
+  }
+
+}
diff --git a/core/modules/serialization/serialization.services.yml b/core/modules/serialization/serialization.services.yml
index 1e28a28b2c..8ad0206c35 100644
--- a/core/modules/serialization/serialization.services.yml
+++ b/core/modules/serialization/serialization.services.yml
@@ -74,7 +74,21 @@ services:
   serializer.normalizer.map:
     class: Drupal\serialization\Normalizer\MapNormalizer
     tags:
-      - { name: normalizer, priority: 25}
+    # This normalizer must be higher than serializer.normalizer.complex_data so
+    # that serializer.normalizer.complex_data is not used for Map objects that
+    # do not provide property definitions. While giving this normalizer a
+    # priority of 1 would work when considering only core's normalizers it would
+    # not allow for any other custom normalizers to given priority between
+    # serializer.normalizer.complex_data and this normalizer. Giving this
+    # normalizer an especially high priority allows normalizers with range of
+    # priorities for other classes that do not provide property definitions.
+    # Giving this normalizer an especially
+    # high priority will NOT cause it to be used for most classes that extend
+    # Map, such any class that extends \Drupal\Core\Field\FieldItemBase,
+    # because those objects will have property definitions.
+    #
+    # @see \Drupal\serialization\Normalizer\MapNormalizer::supportsNormalization
+      - { name: normalizer, priority: 20}
   serializer.encoder.json:
     class: Drupal\serialization\Encoder\JsonEncoder
     tags:
diff --git a/core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldType/MapItem.php b/core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldType/MapItem.php
new file mode 100644
index 0000000000..0cc51cc807
--- /dev/null
+++ b/core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldType/MapItem.php
@@ -0,0 +1,45 @@
+<?php
+
+namespace Drupal\entity_test\Plugin\Field\FieldType;
+
+use Drupal\Core\Field\FieldItemBase;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\Core\TypedData\MapDataDefinition;
+
+/**
+ * Defines the 'map' field type.
+ *
+ * @FieldType(
+ *   id = "map_test",
+ *   label = @Translation("Map Test"),
+ *   description = @Translation("Another dummy field type."),
+ * )
+ */
+class MapItem extends FieldItemBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
+    return [
+      'freeform' => MapDataDefinition::create()->setLabel(t('Freeform')),
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function schema(FieldStorageDefinitionInterface $field_definition) {
+    return [
+      'columns' => [
+        'freeform' => [
+          'description' => 'Serialized array of stuff.',
+          'type' => 'blob',
+          'size' => 'big',
+          'serialize' => TRUE,
+        ],
+      ],
+    ];
+  }
+
+}