diff --git a/core/includes/errors.inc b/core/includes/errors.inc
index 284d1835e9..19676be9f7 100644
--- a/core/includes/errors.inc
+++ b/core/includes/errors.inc
@@ -58,7 +58,9 @@ function drupal_error_levels() {
  *   occurred.
  */
 function _drupal_error_handler_real($error_level, $message, $filename, $line, $context) {
-  if ($error_level & error_reporting()) {
+  // If the site is a test site then fail for user deprecations so they can be
+  // caught by the deprecation error handler.
+  if (($error_level & error_reporting()) || (DRUPAL_TEST_IN_CHILD_SITE && $error_level === E_USER_DEPRECATED)) {
     $types = drupal_error_levels();
     list($severity_msg, $severity_level) = $types[$error_level];
     $backtrace = debug_backtrace();
diff --git a/core/lib/Drupal/Core/Test/HttpClientMiddleware/TestHttpClientMiddleware.php b/core/lib/Drupal/Core/Test/HttpClientMiddleware/TestHttpClientMiddleware.php
index 3c4d1427a8..86e9311d45 100644
--- a/core/lib/Drupal/Core/Test/HttpClientMiddleware/TestHttpClientMiddleware.php
+++ b/core/lib/Drupal/Core/Test/HttpClientMiddleware/TestHttpClientMiddleware.php
@@ -41,7 +41,14 @@ public function __invoke() {
                   // the header.
                   $parameters = unserialize(urldecode($header_value));
                   if (count($parameters) === 3) {
-                    throw new \Exception($parameters[1] . ': ' . $parameters[0] . "\n" . Error::formatBacktrace([$parameters[2]]));
+                    if ($parameters[1] === 'User deprecated function') {
+                      // Fire the same deprecation message to allow it to be
+                      // caught.
+                      @trigger_error((string) $parameters[0], E_USER_DEPRECATED);
+                    }
+                    else {
+                      throw new \Exception($parameters[1] . ': ' . $parameters[0] . "\n" . Error::formatBacktrace([$parameters[2]]));
+                    }
                   }
                   else {
                     throw new \Exception('Error thrown with the wrong amount of parameters.');
diff --git a/core/modules/system/tests/modules/deprecation_test/deprecation_test.module b/core/modules/system/tests/modules/deprecation_test/deprecation_test.module
index fd1092a3c6..e255021d65 100644
--- a/core/modules/system/tests/modules/deprecation_test/deprecation_test.module
+++ b/core/modules/system/tests/modules/deprecation_test/deprecation_test.module
@@ -1,5 +1,10 @@
 <?php
 
+/**
+ * @file
+ * Contains functions for testing calling deprecated functions in tests.
+ */
+
 /**
  * A deprecated function.
  *
diff --git a/core/tests/Drupal/FunctionalTests/Core/Test/PhpUnitBridgeTest.php b/core/tests/Drupal/FunctionalTests/Core/Test/PhpUnitBridgeTest.php
index 6cdb31c103..bc843c31a1 100644
--- a/core/tests/Drupal/FunctionalTests/Core/Test/PhpUnitBridgeTest.php
+++ b/core/tests/Drupal/FunctionalTests/Core/Test/PhpUnitBridgeTest.php
@@ -23,7 +23,7 @@ public function testSilencedError() {
   }
 
   /**
-   * @expectedDeprecation This is the deprecation message for deprecated_test_function().
+   * @expectedDeprecation This is the deprecation message for deprecation_test_function().
    */
   public function testErrorOnSiteUnderTest() {
     $this->drupalGet(Url::fromRoute('deprecation_test.route'));