diff --git a/core/modules/rest/src/Plugin/ResourceBase.php b/core/modules/rest/src/Plugin/ResourceBase.php
index b40a03109b..e91afd9da0 100644
--- a/core/modules/rest/src/Plugin/ResourceBase.php
+++ b/core/modules/rest/src/Plugin/ResourceBase.php
@@ -130,6 +130,10 @@ public function routes() {
             // Expose one route per available format.
             $format_route = clone $route;
             $format_route->addRequirements(['_format' => $format_name]);
+
+            $methods = $route->getMethods();
+            $methods[] = 'POST';
+            $route->setMethods($methods);
             $collection->add("$route_name.$method.$format_name", $format_route);
           }
           break;
diff --git a/core/modules/rest/src/Plugin/rest/resource/EntityResource.php b/core/modules/rest/src/Plugin/rest/resource/EntityResource.php
index 5d9849ded4..68abeea830 100644
--- a/core/modules/rest/src/Plugin/rest/resource/EntityResource.php
+++ b/core/modules/rest/src/Plugin/rest/resource/EntityResource.php
@@ -18,9 +18,11 @@
 use Psr\Log\LoggerInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 use Drupal\rest\ModifiedResourceResponse;
+use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException;
 use Symfony\Component\HttpKernel\Exception\BadRequestHttpException;
+use Symfony\Component\HttpKernel\Exception\ConflictHttpException;
 use Symfony\Component\HttpKernel\Exception\HttpException;
 
 /**
@@ -114,12 +116,19 @@ public static function create(ContainerInterface $container, array $configuratio
    * @param \Drupal\Core\Entity\EntityInterface $entity
    *   The entity object.
    *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The HTTP request.
+   *
    * @return \Drupal\rest\ResourceResponse
    *   The response containing the entity with its accessible fields.
    *
    * @throws \Symfony\Component\HttpKernel\Exception\HttpException
    */
-  public function get(EntityInterface $entity) {
+  public function get(EntityInterface $entity, Request $request) {
+    if ($request->getMethod() === 'POST') {
+      throw new ConflictHttpException('POSTING to an already existing entity resource');
+    }
+
     $entity_access = $entity->access('view', NULL, TRUE);
     if (!$entity_access->isAllowed()) {
       throw new AccessDeniedHttpException($entity_access->getReason() ?: $this->generateFallbackAccessDeniedMessage($entity, 'view'));