diff --git a/core/modules/block/tests/src/Functional/BlockDemoTest.php b/core/modules/block/tests/src/Functional/BlockDemoTest.php
new file mode 100644
index 0000000000000000000000000000000000000000..884b587a11a0fc850fa0df03bebcb31a29c296d7
--- /dev/null
+++ b/core/modules/block/tests/src/Functional/BlockDemoTest.php
@@ -0,0 +1,63 @@
+<?php
+
+namespace Drupal\Tests\block\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests the block demo page with admin themes.
+ *
+ * @group block
+ */
+class BlockDemoTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = ['block'];
+
+  /**
+   * Check for the accessibility of the admin block demo page.
+   */
+  public function testBlockDemo() {
+    // Create administrative user.
+    $admin_user = $this->drupalCreateUser(['administer blocks', 'administer themes']);
+    $this->drupalLogin($admin_user);
+
+    // Confirm we have access to the block demo page for the default theme.
+    $config = $this->container->get('config.factory')->get('system.theme');
+    $default_theme = $config->get('default');
+    $this->drupalGet('admin/structure/block/demo/' . $default_theme);
+    $this->assertResponse(200);
+    $this->assertLinkByHref('admin/structure/block');
+    $this->assertNoLinkByHref('admin/structure/block/list/' . $default_theme);
+
+    // All available themes in core.
+    $available_themes = [
+      'bartik',
+      'classy',
+      'seven',
+      'stark',
+    ];
+
+    // All available themes minute minus the default theme.
+    $themes = array_diff($available_themes, [$default_theme]);
+
+    foreach ($themes as $theme) {
+      // Install theme.
+      $this->container->get('theme_handler')->install([$theme]);
+      // Confirm access to the block demo page for the theme.
+      $this->drupalGet('admin/structure/block/demo/' . $theme);
+      $this->assertResponse(200);
+      // Confirm existence of link for "Exit block region demonstration".
+      $this->assertLinkByHref('admin/structure/block/list/' . $theme);
+    }
+
+    // Confirm access to the block demo page is denied for an invalid theme.
+    $this->drupalGet('admin/structure/block/demo/invalid_theme');
+    $this->assertResponse(403);
+  }
+
+}