diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index 4869591ce6f..bbe95f48fac 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -5,6 +5,7 @@ Yii Framework 2 Change Log
 ------------------------
 
 - Enh #20309: Add custom attributes support to style tags (nzwz)
+- Bug #20239: Fix `yii\data\ActiveDataProvider` to avoid unexpected pagination results with UNION queries (Izumi-kun)
 
 
 2.0.52 February 13, 2025
diff --git a/framework/data/ActiveDataProvider.php b/framework/data/ActiveDataProvider.php
index 3a129aa7293..b79b6fcb54a 100644
--- a/framework/data/ActiveDataProvider.php
+++ b/framework/data/ActiveDataProvider.php
@@ -11,6 +11,7 @@
 use yii\base\Model;
 use yii\db\ActiveQueryInterface;
 use yii\db\Connection;
+use yii\db\Query;
 use yii\db\QueryInterface;
 use yii\di\Instance;
 
@@ -93,14 +94,51 @@ public function init()
     }
 
     /**
-     * {@inheritdoc}
+     * Creates a wrapper of [[query]] that allows adding limit and order.
+     * @return QueryInterface
+     * @throws InvalidConfigException
      */
-    protected function prepareModels()
+    protected function createQueryWrapper(): QueryInterface
     {
         if (!$this->query instanceof QueryInterface) {
             throw new InvalidConfigException('The "query" property must be an instance of a class that implements the QueryInterface e.g. yii\db\Query or its subclasses.');
         }
-        $query = clone $this->query;
+        if (!$this->query instanceof Query || empty($this->query->union)) {
+            return clone $this->query;
+        }
+
+        $wrapper = new class extends Query {
+            /**
+             * @var Query
+             */
+            public $wrappedQuery;
+            /**
+             * @inheritDoc
+             */
+            public function all($db = null)
+            {
+                return $this->wrappedQuery->populate(parent::all($db));
+            }
+            public function createCommand($db = null)
+            {
+                $command = $this->wrappedQuery->createCommand($db);
+                $this->from(['q' => "({$command->getSql()})"])->params($command->params);
+                return parent::createCommand($command->db);
+            }
+        };
+        $wrapper->select('*');
+        $wrapper->wrappedQuery = $this->query;
+        $wrapper->emulateExecution = $this->query->emulateExecution;
+
+        return $wrapper;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function prepareModels()
+    {
+        $query = $this->createQueryWrapper();
         if (($pagination = $this->getPagination()) !== false) {
             $pagination->totalCount = $this->getTotalCount();
             if ($pagination->totalCount === 0) {
@@ -161,11 +199,7 @@ protected function prepareKeys($models)
      */
     protected function prepareTotalCount()
     {
-        if (!$this->query instanceof QueryInterface) {
-            throw new InvalidConfigException('The "query" property must be an instance of a class that implements the QueryInterface e.g. yii\db\Query or its subclasses.');
-        }
-        $query = clone $this->query;
-        return (int) $query->limit(-1)->offset(-1)->orderBy([])->count('*', $this->db);
+        return (int) $this->createQueryWrapper()->limit(-1)->offset(-1)->orderBy([])->count('*', $this->db);
     }
 
     /**
diff --git a/tests/framework/data/ActiveDataProviderTest.php b/tests/framework/data/ActiveDataProviderTest.php
index af96dfef267..c3dd6688f87 100644
--- a/tests/framework/data/ActiveDataProviderTest.php
+++ b/tests/framework/data/ActiveDataProviderTest.php
@@ -198,4 +198,30 @@ public function testDoesNotPerformQueryWhenHasNoModels()
 
         $this->assertEquals(0, $pagination->getPageCount());
     }
+
+    public function testPaginationWithUnionQuery()
+    {
+        $q1 = Item::find()->where(['category_id' => 2])->with('category');
+        $q2 = Item::find()->where(['id' => [2, 4]]);
+        $provider = new ActiveDataProvider([
+            'query' => $q1->union($q2)->indexBy('id'),
+        ]);
+        $pagination = $provider->getPagination();
+        $pagination->pageSize = 2;
+        $provider->prepare();
+        $this->assertEquals(2, $pagination->getPageCount());
+        $this->assertEquals(4, $provider->getTotalCount());
+        $this->assertCount(2, $provider->getModels());
+
+        $pagination->pageSize = 10;
+        $provider->prepare(true);
+        /** @var Item[] $models */
+        $models = $provider->getModels();
+        $this->assertCount(4, $models);
+        $this->assertContainsOnlyInstancesOf(Item::class, $models);
+        $this->assertEquals('Yii 1.1 Application Development Cookbook', $models[2]->name);
+        $this->assertEquals('Toy Story', $models[4]->name);
+        $this->assertTrue($models[2]->isRelationPopulated('category'));
+        $this->assertTrue($models[4]->isRelationPopulated('category'));
+    }
 }