diff -u b/jsonapi.services.yml b/jsonapi.services.yml
--- b/jsonapi.services.yml
+++ b/jsonapi.services.yml
@@ -132,7 +132,7 @@
   # Event subscribers.
   jsonapi.resource_response.subscriber:
     class: Drupal\jsonapi\EventSubscriber\ResourceResponseSubscriber
-    arguments: ['@serializer', '@renderer', '@logger.channel.jsonapi', '@module_handler']
+    arguments: ['@serializer', '@renderer', '@logger.channel.jsonapi', '@module_handler', '@app.root']
     calls:
       - [setValidator, []]
       - [setSchemaFactory, ['@?schemata.schema_factory']] # This is only injected when the service is available.
diff -u b/src/EventSubscriber/ResourceResponseSubscriber.php b/src/EventSubscriber/ResourceResponseSubscriber.php
--- b/src/EventSubscriber/ResourceResponseSubscriber.php
+++ b/src/EventSubscriber/ResourceResponseSubscriber.php
@@ -88,4 +88,11 @@
 
   /**
+   * The application's root file path.
+   *
+   * @var string
+   */
+  protected $app_root;
+
+  /**
    * Constructs a ResourceResponseSubscriber object.
    *
@@ -97,12 +104,15 @@
    *   The JSON API logger channel.
    * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
    *   The module handler.
+   * @param string $app_root
+   *   The application's root file path.
    */
-  public function __construct(SerializerInterface $serializer, RendererInterface $renderer, LoggerInterface $logger, ModuleHandlerInterface $module_handler) {
+  public function __construct(SerializerInterface $serializer, RendererInterface $renderer, LoggerInterface $logger, ModuleHandlerInterface $module_handler, $app_root) {
     $this->serializer = $serializer;
     $this->renderer = $renderer;
     $this->logger = $logger;
     $this->moduleHandler = $module_handler;
+    $this->appRoot = $app_root;
   }
 
   /**
@@ -268,7 +278,7 @@
 
     $schema_ref = sprintf(
       'file://%s/%s/%s',
-      DRUPAL_ROOT,
+      $this->appRoot,
       $this->moduleHandler->getModule('jsonapi')->getPath(),
       'schema.json'
     );
diff -u b/tests/src/Unit/EventSubscriber/ResourceResponseSubscriberTest.php b/tests/src/Unit/EventSubscriber/ResourceResponseSubscriberTest.php
--- b/tests/src/Unit/EventSubscriber/ResourceResponseSubscriberTest.php
+++ b/tests/src/Unit/EventSubscriber/ResourceResponseSubscriberTest.php
@@ -17,8 +17,6 @@
 use Symfony\Component\Routing\Route;
 use Symfony\Component\Serializer\Serializer;
 
-define('DRUPAL_ROOT', dirname(dirname(dirname(dirname(dirname(__DIR__))))));
-
 /**
  * @coversDefaultClass \Drupal\jsonapi\EventSubscriber\ResourceResponseSubscriber
  * @group jsonapi
@@ -36,6 +34,7 @@
    * {@inheritdoc}
    */
   public function setUp() {
+    parent::setUp();
     // Check that the validation class is available.
     if (!class_exists("\\JsonSchema\\Validator")) {
       $this->fail('The JSON Schema validator is missing. You can install it with `composer require justinrainbow/json-schema`.');
@@ -43,15 +42,15 @@
 
     $module_handler = $this->prophesize(ModuleHandlerInterface::class);
     $module = $this->prophesize(Extension::class);
-    $module->getPath()->willReturn('jsonapi');
-    $module_handler->getModule('jsonapi')->willReturn(
-      $module->reveal()
-    );
+    $module_path = substr(dirname(dirname(dirname(__DIR__))), 0, strlen($this->root));
+    $module->getPath()->willReturn($module_path);
+    $module_handler->getModule('jsonapi')->willReturn($module->reveal());
     $subscriber = new ResourceResponseSubscriber(
       new Serializer([], [new JsonSchemaEncoder()]),
       $this->prophesize(RendererInterface::class)->reveal(),
       $this->prophesize(LoggerInterface::class)->reveal(),
-      $module_handler->reveal()
+      $module_handler->reveal(),
+      $this->root
     );
     $subscriber->setValidator();
     $this->subscriber = $subscriber;