diff --git a/core/modules/forum/forum.install b/core/modules/forum/forum.install
index 7b6f40f9bac..8ec203f14fd 100644
--- a/core/modules/forum/forum.install
+++ b/core/modules/forum/forum.install
@@ -161,6 +161,7 @@ function forum_schema() {
       'created' => ['created'],
       'last_comment_timestamp' => ['last_comment_timestamp'],
     ],
+    'primary key' => ['nid', 'tid'],
     'foreign keys' => [
       'tracked_node' => [
         'table' => 'node',
@@ -204,3 +205,15 @@ function forum_update_10100(&$sandbox = NULL) {
     $connection->schema()->changeField('forum_index', 'last_comment_timestamp', 'last_comment_timestamp', $new);
   }
 }
+
+/**
+ * Add a primary key to forum_index.
+ */
+function forum_update_10101(&$sandbox = NULL) {
+  $connection = \Drupal::database();
+  if ($connection->schema()->tableExists('forum_index')) {
+    $connection->schema()->addPrimaryKey('forum_index', ['nid', 'tid']);
+    return \t('Added primary key to the forum_index table.');
+  }
+  return \t('The forum_index table does not exist, the update was skipped.');
+}
diff --git a/core/modules/forum/tests/fixtures/update/drupal-10.1.0.empty.standard.forum.gz b/core/modules/forum/tests/fixtures/update/drupal-10.1.0.empty.standard.forum.gz
new file mode 100644
index 00000000000..47d39d11298
Binary files /dev/null and b/core/modules/forum/tests/fixtures/update/drupal-10.1.0.empty.standard.forum.gz differ
diff --git a/core/modules/forum/tests/src/Functional/ForumIndexUpdateTest.php b/core/modules/forum/tests/src/Functional/ForumIndexUpdateTest.php
new file mode 100644
index 00000000000..70a719a89d3
--- /dev/null
+++ b/core/modules/forum/tests/src/Functional/ForumIndexUpdateTest.php
@@ -0,0 +1,35 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\forum\Functional;
+
+use Drupal\FunctionalTests\Update\UpdatePathTestBase;
+
+/**
+ * Tests addition of the forum_index primary key.
+ *
+ * @group forum
+ */
+final class ForumIndexUpdateTest extends UpdatePathTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setDatabaseDumpFiles() {
+    $this->databaseDumpFiles = [
+      dirname(__DIR__, 2) . '/fixtures/update/drupal-10.1.0.empty.standard.forum.gz',
+    ];
+  }
+
+  /**
+   * Tests the update path to add the new primary key.
+   */
+  public function testUpdatePath(): void {
+    $schema = \Drupal::database()->schema();
+    $this->assertFalse($schema->indexExists('forum_index', 'PRIMARY'));
+    $this->runUpdates();
+    $this->assertTrue($schema->indexExists('forum_index', 'PRIMARY'));
+  }
+
+}
diff --git a/core/modules/forum/tests/src/Kernel/ForumIndexTest.php b/core/modules/forum/tests/src/Kernel/ForumIndexTest.php
new file mode 100644
index 00000000000..3c5def9853e
--- /dev/null
+++ b/core/modules/forum/tests/src/Kernel/ForumIndexTest.php
@@ -0,0 +1,52 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\forum\Kernel;
+
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Defines a class for testing the forum_index table.
+ *
+ * @group forum
+ */
+final class ForumIndexTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'system',
+    'user',
+    'node',
+    'history',
+    'taxonomy',
+    'forum',
+    'comment',
+    'options',
+    'text',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->installEntitySchema('node');
+    $this->installEntitySchema('user');
+    $this->installEntitySchema('comment');
+    $this->installEntitySchema('taxonomy_term');
+    $this->installSchema('forum', ['forum_index']);
+  }
+
+  /**
+   * Tests there's a primary key on the forum_index table.
+   */
+  public function testForumIndexIndex(): void {
+    $schema = \Drupal::database()->schema();
+    $this->assertTrue($schema->tableExists('forum_index'));
+    $this->assertTrue($schema->indexExists('forum_index', 'PRIMARY'));
+  }
+
+}