diff --cc core/modules/hal/tests/src/Functional/EntityResource/HalEntityNormalizationTrait.php
index cefe743,5576b2b..0000000
--- a/core/modules/hal/tests/src/Functional/EntityResource/HalEntityNormalizationTrait.php
+++ b/core/modules/hal/tests/src/Functional/EntityResource/HalEntityNormalizationTrait.php
@@@ -101,18 -101,26 +101,18 @@@ trait HalEntityNormalizationTrait 
        $normalization['_links']['type'] = Url::fromUri('base:rest/type/' . static::$entityTypeId . '/bad_bundle_name');
        $request_options[RequestOptions::BODY] = $this->serializer->encode($normalization, static::$format);
  
--      // DX: 400 when incorrect entity type bundle is specified.
++      // DX: 422 when incorrect entity type bundle is specified.
        $response = $this->request($method, $url, $request_options);
-       $this->assertResourceErrorResponse(400, 'No entity type(s) specified', $response);
 -      // @todo Uncomment, remove next 3 in https://www.drupal.org/node/2813853.
 -      // $this->assertResourceErrorResponse(400, 'No entity type(s) specified', $response);
 -      $this->assertSame(400, $response->getStatusCode());
 -      $this->assertSame([static::$mimeType], $response->getHeader('Content-Type'));
 -      $this->assertSame($this->serializer->encode(['error' => 'No entity type(s) specified'], static::$format), (string) $response->getBody());
++      $this->assertResourceErrorResponse(422, 'No entity type(s) specified', $response);
  
  
        unset($normalization['_links']['type']);
        $request_options[RequestOptions::BODY] = $this->serializer->encode($normalization, static::$format);
  
  
-       // DX: 400 when no entity type bundle is specified.
+       // DX: 422 when no entity type bundle is specified.
        $response = $this->request($method, $url, $request_options);
-       $this->assertResourceErrorResponse(400, 'The type link relation must be specified.', $response);
 -      // @todo Uncomment, remove next 3 in https://www.drupal.org/node/2813853.
 -      // $this->assertResourceErrorResponse(422, 'The type link relation must be specified.', $response);
 -      $this->assertSame(422, $response->getStatusCode());
 -      $this->assertSame([static::$mimeType], $response->getHeader('Content-Type'));
 -      $this->assertSame($this->serializer->encode(['error' => 'The type link relation must be specified.'], static::$format), (string) $response->getBody());
++      $this->assertResourceErrorResponse(422, 'The type link relation must be specified.', $response);
      }
    }
  
diff --cc core/modules/rest/src/RequestHandler.php
index 0e9f41b,8857c2e..0000000
--- a/core/modules/rest/src/RequestHandler.php
+++ b/core/modules/rest/src/RequestHandler.php
@@@ -10,9 -10,10 +10,11 @@@ use Symfony\Component\DependencyInjecti
  use Symfony\Component\DependencyInjection\ContainerAwareTrait;
  use Symfony\Component\DependencyInjection\ContainerInterface;
  use Symfony\Component\HttpFoundation\Request;
 -use Symfony\Component\HttpKernel\Exception\HttpException;
 +use Symfony\Component\HttpKernel\Exception\BadRequestHttpException;
++use Symfony\Component\HttpKernel\Exception\UnprocessableEntityHttpException;
  use Symfony\Component\HttpKernel\Exception\UnsupportedMediaTypeHttpException;
  use Symfony\Component\Serializer\Exception\UnexpectedValueException;
+ use Symfony\Component\Serializer\Exception\InvalidArgumentException;
  
  /**
   * Acts as intermediate request forwarder for resource plugins.
@@@ -96,19 -97,32 +98,32 @@@ class RequestHandler implements Contain
        $request_method = $request->getMethod();
        if (in_array($format, $resource_config->getFormats($request_method))) {
          $definition = $resource->getPluginDefinition();
+ 
+         // First decode the request data. We can then determine if the
+         // serialized data was malformed.
          try {
-           if (!empty($definition['serialization_class'])) {
-             $unserialized = $serializer->deserialize($received, $definition['serialization_class'], $format, array('request_method' => $method));
-           }
-           // If the plugin does not specify a serialization class just decode
-           // the received data.
-           else {
-             $unserialized = $serializer->decode($received, $format, array('request_method' => $method));
-           }
+           $unserialized = $serializer->decode($received, $format, ['request_method' => $method]);
          }
          catch (UnexpectedValueException $e) {
+           // If an exception was thrown at this stage, there was a problem
+           // decoding the data. Throw a 400 http exception.
 -          $this->throwHttpException(400, $e, $request);
 +          throw new BadRequestHttpException($e->getMessage());
          }
+ 
+         // Then attempt to denormalize if there is a serialization class.
+         if (!empty($definition['serialization_class'])) {
+           try {
+             $unserialized = $serializer->denormalize($unserialized, $definition['serialization_class'], $format, ['request_method' => $method]);
+           }
+           // These two serialization exception types mean there was a problem
+           // with the structure of the decoded data and it's not valid.
+           catch (UnexpectedValueException $e) {
 -            $this->throwHttpException(422, $e, $request);
++            throw new UnprocessableEntityHttpException($e->getMessage());
+           }
+           catch (InvalidArgumentException $e) {
 -            $this->throwHttpException(422, $e, $request);
++            throw new UnprocessableEntityHttpException($e->getMessage());
+           }
+         }
        }
        else {
          throw new UnsupportedMediaTypeHttpException();
diff --cc core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index e9d9c48,88fed50..0000000
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@@ -960,10 -965,13 +960,9 @@@ abstract class EntityResourceTestBase e
          $request_options[RequestOptions::BODY] = $this->serializer->encode($normalization, static::$format);
  
  
-         // DX: 400 when incorrect entity type bundle is specified.
-         // @todo Change to 422 in https://www.drupal.org/node/2827084.
+         // DX: 422 when incorrect entity type bundle is specified.
          $response = $this->request($method, $url, $request_options);
-         $this->assertResourceErrorResponse(400, '"bad_bundle_name" is not a valid bundle type for denormalization.', $response);
 -        // @todo use this commented line instead of the 3 lines thereafter once https://www.drupal.org/node/2813853 lands.
 -        //      $this->assertResourceErrorResponse(422, '"bad_bundle_name" is not a valid bundle type for denormalization.', $response);
 -        $this->assertSame(422, $response->getStatusCode());
 -        $this->assertSame([static::$mimeType], $response->getHeader('Content-Type'));
 -        $this->assertSame($this->serializer->encode(['error' => '"bad_bundle_name" is not a valid bundle type for denormalization.'], static::$format), (string) $response->getBody());
++        $this->assertResourceErrorResponse(422, '"bad_bundle_name" is not a valid bundle type for denormalization.', $response);
        }
  
  
@@@ -971,10 -979,13 +970,9 @@@
        $request_options[RequestOptions::BODY] = $this->serializer->encode($normalization, static::$format);
  
  
-       // DX: 400 when no entity type bundle is specified.
-       // @todo Change to 422 in https://www.drupal.org/node/2827084.
+       // DX: 422 when no entity type bundle is specified.
        $response = $this->request($method, $url, $request_options);
-       $this->assertResourceErrorResponse(400, 'A string must be provided as a bundle value.', $response);
 -      // @todo use this commented line instead of the 3 lines thereafter once https://www.drupal.org/node/2813853 lands.
 -      // $this->assertResourceErrorResponse(422, 'A string must be provided as a bundle value.', $response);
 -      $this->assertSame(422, $response->getStatusCode());
 -      $this->assertSame([static::$mimeType], $response->getHeader('Content-Type'));
 -      $this->assertSame($this->serializer->encode(['error' => 'A string must be provided as a bundle value.'], static::$format), (string) $response->getBody());
++      $this->assertResourceErrorResponse(422, 'A string must be provided as a bundle value.', $response);
      }
    }