diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index 48d3e0f550..84a1c5b08a 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -54,6 +54,7 @@ Yii Framework 2 Change Log
 - Enh #20591: Add PHPStan/Psalm annotations for `yii\rbac\BaseManager::getItems()` (mspirkov)
 - Bug #20594: Fix `@return` annotation for `Instance::get()` (mspirkov)
 - Bug #20595: Fix `@return` annotation for `BaseHtml::getAttributeValue()` (mspirkov)
+- Bug #20605: Fix return value in `SerialColumn::renderDataCellContent()` (mspirkov)
 - Bug #20604: Fix `@var` annotation for `yii\db\Command::$pdoStatement` (mspirkov)
 - Bug #20600: Fix `@var` annotation for `yii\test\FileFixtureTrait::$dataFile` (mspirkov)
 - Bug #20608: Fix `@return` annotations for `yii\rest\Serializer` methods (mspirkov)
diff --git a/framework/grid/SerialColumn.php b/framework/grid/SerialColumn.php
index b63696e8c9..46c274ec52 100644
--- a/framework/grid/SerialColumn.php
+++ b/framework/grid/SerialColumn.php
@@ -42,9 +42,9 @@ class SerialColumn extends Column
     {
         $pagination = $this->grid->dataProvider->getPagination();
         if ($pagination !== false) {
-            return $pagination->getOffset() + $index + 1;
+            return (string) ($pagination->getOffset() + $index + 1);
         }
 
-        return $index + 1;
+        return (string) ($index + 1);
     }
 }
diff --git a/tests/framework/grid/SerialColumnTest.php b/tests/framework/grid/SerialColumnTest.php
new file mode 100644
index 0000000000..149264ccd6
--- /dev/null
+++ b/tests/framework/grid/SerialColumnTest.php
@@ -0,0 +1,108 @@
+<?php
+
+/**
+ * @link https://www.yiiframework.com/
+ * @copyright Copyright (c) 2008 Yii Software LLC
+ * @license https://www.yiiframework.com/license/
+ */
+
+declare(strict_types=1);
+
+namespace yiiunit\framework\grid;
+
+use Yii;
+use yii\data\ArrayDataProvider;
+use yii\data\Pagination;
+use yii\grid\GridView;
+use yii\grid\SerialColumn;
+
+/**
+ * @group grid
+ */
+class SerialColumnTest extends \yiiunit\TestCase
+{
+    /**
+     * @dataProvider provideRenderDataCellData
+     */
+    public function testRenderDataCell(
+        array $dataProviderConfig,
+        ?string $page,
+        int $index,
+        string $expectedResult
+    ): void {
+        $this->mockWebApplication();
+
+        Yii::$app->getRequest()->setQueryParams([
+            'page' => $page,
+        ]);
+
+        $column = new SerialColumn([
+            'grid' => new GridView([
+                'dataProvider' => new ArrayDataProvider($dataProviderConfig),
+            ]),
+        ]);
+
+        $result = $column->renderDataCell(['id' => 1], 1, $index);
+        $this->assertSame($expectedResult, $result);
+    }
+
+    public static function provideRenderDataCellData(): array
+    {
+        return [
+            [
+                [
+                    'pagination' => false,
+                ],
+                null,
+                0,
+                '<td>1</td>',
+            ],
+            [
+                [
+                    'pagination' => false,
+                ],
+                null,
+                5,
+                '<td>6</td>',
+            ],
+            [
+                [
+                    'pagination' => new Pagination(),
+                ],
+                null,
+                0,
+                '<td>1</td>',
+            ],
+            [
+                [
+                    'pagination' => new Pagination(),
+                ],
+                null,
+                4,
+                '<td>5</td>',
+            ],
+            [
+                [
+                    'pagination' => new Pagination([
+                        'totalCount' => 20,
+                        'defaultPageSize' => 10,
+                    ]),
+                ],
+                '2',
+                0,
+                '<td>11</td>',
+            ],
+            [
+                [
+                    'pagination' => new Pagination([
+                        'totalCount' => 20,
+                        'defaultPageSize' => 10,
+                    ]),
+                ],
+                '2',
+                3,
+                '<td>14</td>',
+            ],
+        ];
+    }
+}