diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index 7c0206b081..36fbda0690 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -3,7 +3,6 @@ Yii Framework 2 Change Log
 
 2.0.7 under development
 -----------------------
-
 - Bug #6351: Find MySQL FK constraints from `information_schema` tables instead of `SHOW CREATE TABLE` to improve reliability (nineinchnick)
 - Bug #6363, #8301, #8582, #9566: Fixed data methods and PJAX issues when used together (derekisbusy)
 - Bug #6876: Fixed RBAC migration MSSQL cascade problem (thejahweh)
@@ -43,6 +42,7 @@ Yii Framework 2 Change Log
 - Bug #10029: Fixed MaskedInput not working with PJAX (martrix78, samdark)
 - Bug #10101: Fixed assignments saving on role removing in `\yii\rbac\PhpManager` (rezident1307)
 - Bug #10142: Fixed `yii\validators\EmailValidator` to check the length of email properly (silverfire)
+- Bug #10263: Fixed `yii\validators\UniqueValidator` to work properly when model is not instance of `targetClass` (bupy7, githubjeka, silverfire)
 - Bug #10278: Fixed `yii\helpers\BaseJson` support \SimpleXMLElement data (SilverFire, LAV45)
 - Bug #10302: Fixed JS function `yii.getQueryParams`, which parsed array variables incorrectly (servocoder, silverfire)
 - Bug #10363: Fixed fallback float and integer formatting (AnatolyRugalev, silverfire)
diff --git a/framework/validators/UniqueValidator.php b/framework/validators/UniqueValidator.php
index 19882c607f..a15b4cb110 100644
--- a/framework/validators/UniqueValidator.php
+++ b/framework/validators/UniqueValidator.php
@@ -106,8 +106,9 @@ class UniqueValidator extends Validator
             $query->andWhere($this->filter);
         }
 
-        if (!$model instanceof ActiveRecordInterface || $model->getIsNewRecord()) {
+        if (!$model instanceof ActiveRecordInterface || $model->getIsNewRecord() || $model->className() !== $targetClass) {
             // if current $model isn't in the database yet then it's OK just to call exists()
+            // also there's no need to run check based on primary keys, when $targetClass is not the same as $model's class
             $exists = $query->exists();
         } else {
             // if current $model is in the database already we can't use exists()
diff --git a/tests/data/validators/models/ValidatorTestRefModel.php b/tests/data/validators/models/ValidatorTestRefModel.php
index bf0ebe5dbd..bd70734350 100644
--- a/tests/data/validators/models/ValidatorTestRefModel.php
+++ b/tests/data/validators/models/ValidatorTestRefModel.php
@@ -4,6 +4,11 @@ namespace yiiunit\data\validators\models;
 
 use yiiunit\data\ar\ActiveRecord;
 
+/**
+ * @property int id
+ * @property string a_field
+ * @property int ref
+ */
 class ValidatorTestRefModel extends ActiveRecord
 {
 
diff --git a/tests/framework/validators/UniqueValidatorTest.php b/tests/framework/validators/UniqueValidatorTest.php
index 7a1b805725..53990a63f8 100644
--- a/tests/framework/validators/UniqueValidatorTest.php
+++ b/tests/framework/validators/UniqueValidatorTest.php
@@ -5,8 +5,10 @@ namespace yiiunit\framework\validators;
 use yii\validators\UniqueValidator;
 use Yii;
 use yiiunit\data\ar\ActiveRecord;
+use yiiunit\data\ar\Customer;
 use yiiunit\data\ar\Order;
 use yiiunit\data\ar\OrderItem;
+use yiiunit\data\ar\Profile;
 use yiiunit\data\validators\models\FakedValidationModel;
 use yiiunit\data\validators\models\ValidatorTestMainModel;
 use yiiunit\data\validators\models\ValidatorTestRefModel;
@@ -38,6 +40,7 @@ class UniqueValidatorTest extends DatabaseTestCase
         $m = ValidatorTestMainModel::find()->one();
         $val->validateAttribute($m, 'id');
         $this->assertFalse($m->hasErrors('id'));
+        /** @var ValidatorTestRefModel $m */
         $m = ValidatorTestRefModel::findOne(1);
         $val->validateAttribute($m, 'ref');
         $this->assertTrue($m->hasErrors('ref'));
@@ -73,6 +76,7 @@ class UniqueValidatorTest extends DatabaseTestCase
     public function testValidateNonDatabaseAttribute()
     {
         $val = new UniqueValidator(['targetClass' => ValidatorTestRefModel::className(), 'targetAttribute' => 'ref']);
+        /** @var ValidatorTestMainModel $m */
         $m = ValidatorTestMainModel::findOne(1);
         $val->validateAttribute($m, 'testMainVal');
         $this->assertFalse($m->hasErrors('testMainVal'));
@@ -97,6 +101,7 @@ class UniqueValidatorTest extends DatabaseTestCase
             'targetAttribute' => ['order_id', 'item_id'],
         ]);
         // validate old record
+        /** @var OrderItem $m */
         $m = OrderItem::findOne(['order_id' => 1, 'item_id' => 2]);
         $val->validateAttribute($m, 'order_id');
         $this->assertFalse($m->hasErrors('order_id'));
@@ -117,6 +122,7 @@ class UniqueValidatorTest extends DatabaseTestCase
             'targetAttribute' => ['id' => 'order_id'],
         ]);
         // validate old record
+        /** @var Order $m */
         $m = Order::findOne(1);
         $val->validateAttribute($m, 'id');
         $this->assertTrue($m->hasErrors('id'));
@@ -136,4 +142,33 @@ class UniqueValidatorTest extends DatabaseTestCase
         $val->validateAttribute($m, 'id');
         $this->assertFalse($m->hasErrors('id'));
     }
+
+    public function testValidateTargetClass()
+    {
+        // Check whether "Description" and "address" aren't equal
+        $val = new UniqueValidator([
+            'targetClass' => Customer::className(),
+            'targetAttribute' => ['description'=>'address'],
+        ]);
+
+        /** @var Profile $m */
+        $m = Profile::findOne(1);
+        $this->assertEquals('profile customer 1', $m->description);
+        $val->validateAttribute($m, 'description');
+        $this->assertFalse($m->hasErrors('description'));
+
+        // ID of Profile is not equal to ID of Customer
+        // (1, description = address2) <=> (2, address = address2)
+        $m->description = 'address2';
+        $val->validateAttribute($m, 'description');
+        $this->assertTrue($m->hasErrors('description'));
+        $m->clearErrors('description');
+
+        // ID of Profile IS equal to ID of Customer
+        // (1, description = address1) <=> (1, address = address1)
+        // https://github.com/yiisoft/yii2/issues/10263
+        $m->description = 'address1';
+        $val->validateAttribute($m, 'description');
+        $this->assertTrue($m->hasErrors('description'));
+    }
 }