diff --git a/core/modules/user/tests/src/Kernel/UserWhosOnlineBlockTest.php b/core/modules/user/tests/src/Kernel/UserWhosOnlineBlockTest.php
new file mode 100644
index 0000000000..b5778e6cea
--- /dev/null
+++ b/core/modules/user/tests/src/Kernel/UserWhosOnlineBlockTest.php
@@ -0,0 +1,117 @@
+<?php
+
+namespace Drupal\Tests\user\Kernel;
+
+use Drupal\KernelTests\KernelTestBase;
+use Drupal\user\Entity\User;
+
+/**
+ * Tests the 'Whos Online Block'.
+ *
+ * @group user
+ */
+class UserWhosOnlineBlockTest extends KernelTestBase {
+
+  /**
+   * The block being tested.
+   *
+   * @var \Drupal\block\Entity\BlockInterface
+   */
+  protected $block;
+
+  /**
+   * The block storage.
+   *
+   * @var \Drupal\Core\Config\Entity\ConfigEntityStorageInterface
+   */
+  protected $controller;
+
+  /**
+   * The renderer.
+   *
+   * @var \Drupal\Core\Render\RendererInterface
+   */
+  protected $renderer;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['system', 'user', 'block', 'views'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->installConfig(['system', 'block', 'views', 'user']);
+    $this->installSchema('system', ['sequences']);
+    $this->installEntitySchema('user');
+
+    $this->controller = $this->container
+      ->get('entity_type.manager')
+      ->getStorage('block');
+
+    // Create a block with only required values.
+    $this->block = $this->controller->create([
+      'plugin' => 'views_block:who_s_online-who_s_online_block',
+      'region' => 'sidebar_first',
+      'id' => strtolower($this->randomMachineName(8)),
+      'theme' => \Drupal::configFactory()->get('system.theme')->get('default'),
+      'label' => $this->randomMachineName(8),
+      'visibility' => [],
+      'weight' => 0,
+    ]);
+
+    $this->block->save();
+
+    $this->container->get('cache.render')->deleteAll();
+
+    $this->renderer = $this->container->get('renderer');
+  }
+
+  /**
+   * Test the Who's Online block.
+   */
+  public function testWhosOnlineBlock() {
+
+    // Generate users.
+    $user1 = User::create([
+      'name' => 'user1',
+      'mail' => 'user1example.com',
+    ]);
+    $user1->addRole('administrator');
+    $user1->activate();
+    $user1->setLastAccessTime(REQUEST_TIME);
+    $user1->save();
+
+    $user2 = User::create([
+      'name' => 'user2',
+      'mail' => 'user2example.com',
+    ]);
+    $user2->activate();
+    $user2->setLastAccessTime(REQUEST_TIME + 1);
+    $user2->save();
+
+    $user3 = User::create([
+      'name' => 'user3',
+      'mail' => 'user2example.com',
+    ]);
+    $user3->activate();
+    // Insert an inactive user who should not be seen in the block, and ensure
+    // that the admin user used in setUp() does not appear.
+    $inactive_time = REQUEST_TIME - (15 * 60) - 1;
+    $user3->setLastAccessTime($inactive_time);
+    $user3->save();
+
+    // Test block output.
+    \Drupal::currentUser()->setAccount($user1);
+    $content = entity_view($this->block, 'block');
+    $this->setRawContent($this->renderer->renderRoot($content));
+    $this->assertRaw('2 users', 'Correct number of online users (2 users).');
+    $this->assertText($user1->getUsername(), 'Active user 1 found in online list.');
+    $this->assertText($user2->getUsername(), 'Active user 2 found in online list.');
+    $this->assertNoText($user3->getUsername(), 'Inactive user not found in online list.');
+    $this->assertTrue(strpos($this->getRawContent(), $user1->getUsername()) > strpos($this->getRawContent(), $user2->getUsername()), 'Online users are ordered correctly.');
+  }
+
+}