diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index 894b1ad2a5d..dff044e9640 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -4,6 +4,7 @@ Yii Framework 2 Change Log
 2.0.49 under development
 ------------------------
 
+- Enh #12743: Added new methods `BaseActiveRecord::loadRelations()` and `BaseActiveRecord::loadRelationsFor()` to eager load related models for existing primary model instances (PowerGamer1)
 - Bug #19857: Fix AttributeTypecastBehavior::resetOldAttributes() causes "class has no attribute named" InvalidArgumentException (uaoleg)
 - Bug #18859: Fix `yii\web\Controller::bindInjectedParams()` to not throw error when argument of `ReflectionUnionType` type is passed (bizley)
 - Enh #19841: Allow jQuery 3.7 to be installed (wouter90)
diff --git a/framework/db/BaseActiveRecord.php b/framework/db/BaseActiveRecord.php
index 88fb2f11d7c..3a67ac4100c 100644
--- a/framework/db/BaseActiveRecord.php
+++ b/framework/db/BaseActiveRecord.php
@@ -1781,4 +1781,57 @@ private function isValueDifferent($newValue, $oldValue)
 
         return $newValue !== $oldValue;
     }
+
+    /**
+     * Eager loads related models for the already loaded primary models.
+     *
+     * Helps to reduce the number of queries performed against database if some related models are only used
+     * when a specific condition is met. For example:
+     *
+     * ```php
+     * $customers = Customer::find()->where(['country_id' => 123])->all();
+     * if (Yii:app()->getUser()->getIdentity()->canAccessOrders()) {
+     *     Customer::loadRelationsFor($customers, 'orders.items');
+     * }
+     * ```
+     *
+     * @param array|ActiveRecordInterface[] $models array of primary models. Each model should have the same type and can be:
+     * - an active record instance;
+     * - active record instance represented by array (i.e. active record was loaded using [[ActiveQuery::asArray()]]).
+     * @param string|array $relationNames the names of the relations of primary models to be loaded from database. See [[ActiveQueryInterface::with()]] on how to specify this argument.
+     * @param bool $asArray whether to load each related model as an array or an object (if the relation itself does not specify that).
+     * @since 2.0.49
+     */
+    public static function loadRelationsFor(&$models, $relationNames, $asArray = false)
+    {
+        // ActiveQueryTrait::findWith() called below assumes $models array is non-empty.
+        if (empty($models)) {
+            return;
+        }
+
+        static::find()->asArray($asArray)->findWith((array)$relationNames, $models);
+    }
+
+    /**
+     * Eager loads related models for the already loaded primary model.
+     *
+     * Helps to reduce the number of queries performed against database if some related models are only used
+     * when a specific condition is met. For example:
+     *
+     * ```php
+     * $customer = Customer::find()->where(['id' => 123])->one();
+     * if (Yii:app()->getUser()->getIdentity()->canAccessOrders()) {
+     *     $customer->loadRelations('orders.items');
+     * }
+     * ```
+     *
+     * @param string|array $relationNames the names of the relations of this model to be loaded from database. See [[ActiveQueryInterface::with()]] on how to specify this argument.
+     * @param bool $asArray whether to load each relation as an array or an object (if the relation itself does not specify that).
+     * @since 2.0.49
+     */
+    public function loadRelations($relationNames, $asArray = false)
+    {
+        $models = [$this];
+        static::loadRelationsFor($models, $relationNames, $asArray);
+    }
 }
diff --git a/tests/framework/db/ActiveRecordTest.php b/tests/framework/db/ActiveRecordTest.php
index f1370f47282..9ddd9a0ad87 100644
--- a/tests/framework/db/ActiveRecordTest.php
+++ b/tests/framework/db/ActiveRecordTest.php
@@ -2192,4 +2192,50 @@ public function testVirtualRelation()
         $this->assertNotNull($order->virtualCustomer);
     }
 
+    public function testLoadRelations()
+    {
+        // Test eager loading relations for multiple primary models using loadRelationsFor().
+        /** @var Customer[] $customers */
+        $customers = Customer::find()->all();
+        Customer::loadRelationsFor($customers, ['orders.items']);
+        foreach ($customers as $customer) {
+            $this->assertTrue($customer->isRelationPopulated('orders'));
+            foreach ($customer->orders as $order) {
+                $this->assertTrue($order->isRelationPopulated('items'));
+            }
+        }
+
+        // Test eager loading relations as arrays.
+        /** @var array $customers */
+        $customers = Customer::find()->asArray(true)->all();
+        Customer::loadRelationsFor($customers, ['orders.items' => function ($query) { $query->asArray(false); }], true);
+        foreach ($customers as $customer) {
+            $this->assertTrue(isset($customer['orders']));
+            $this->assertTrue(is_array($customer['orders']));
+            foreach ($customer['orders'] as $order) {
+                $this->assertTrue(is_array($order));
+                $this->assertTrue(isset($order['items']));
+                $this->assertTrue(is_array($order['items']));
+                foreach ($order['items'] as $item) {
+                    $this->assertFalse(is_array($item));
+                }
+            }
+        }
+
+        // Test eager loading relations for a single primary model using loadRelations().
+        /** @var Customer $customer */
+        $customer = Customer::find()->where(['id' => 1])->one();
+        $customer->loadRelations('orders.items');
+        $this->assertTrue($customer->isRelationPopulated('orders'));
+        foreach ($customer->orders as $order) {
+            $this->assertTrue($order->isRelationPopulated('items'));
+        }
+
+        // Test eager loading previously loaded relation (relation value should be replaced with a new value loaded from database).
+        /** @var Customer $customer */
+        $customer = Customer::find()->where(['id' => 2])->with(['orders' => function ($query) { $query->orderBy(['id' => SORT_ASC]); }])->one();
+        $this->assertTrue($customer->orders[0]->id < $customer->orders[1]->id, 'Related models should be sorted by ID in ascending order.');
+        $customer->loadRelations(['orders' => function ($query) { $query->orderBy(['id' => SORT_DESC]); }]);
+        $this->assertTrue($customer->orders[0]->id > $customer->orders[1]->id, 'Related models should be sorted by ID in descending order.');
+    }
 }