diff --git a/app/code/Magento/GroupedProduct/Api/Data/GroupedOptionsInterface.php b/app/code/Magento/GroupedProduct/Api/Data/GroupedOptionsInterface.php
new file mode 100644
index 0000000000000..9278ef168eb0d
--- /dev/null
+++ b/app/code/Magento/GroupedProduct/Api/Data/GroupedOptionsInterface.php
@@ -0,0 +1,45 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\GroupedProduct\Api\Data;
+
+use Magento\Framework\Api\ExtensibleDataInterface;
+
+/**
+ * Represents `product item id with qty` of a grouped product.
+ */
+interface GroupedOptionsInterface extends ExtensibleDataInterface
+{
+    /**
+     * Get associated product id
+     *
+     * @return int|null
+     */
+    public function getId(): ?int;
+
+    /**
+     * Get associated product qty
+     *
+     * @return int|null
+     */
+    public function getQty(): ?int;
+
+    /**
+     * Set extension attributes
+     *
+     * @param \Magento\GroupedProduct\Api\Data\GroupedOptionsExtensionInterface $extensionAttributes
+     * @return void
+     */
+    public function setExtensionAttributes(GroupedOptionsExtensionInterface $extensionAttributes): void;
+
+    /**
+     * Get extension attributes
+     *
+     * @return \Magento\GroupedProduct\Api\Data\GroupedOptionsExtensionInterface|null
+     */
+    public function getExtensionAttributes(): ?GroupedOptionsExtensionInterface;
+}
diff --git a/app/code/Magento/GroupedProduct/Model/Quote/Item/CartItemProcessor.php b/app/code/Magento/GroupedProduct/Model/Quote/Item/CartItemProcessor.php
new file mode 100644
index 0000000000000..78ab91194992e
--- /dev/null
+++ b/app/code/Magento/GroupedProduct/Model/Quote/Item/CartItemProcessor.php
@@ -0,0 +1,124 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\GroupedProduct\Model\Quote\Item;
+
+use Magento\Framework\DataObject;
+use Magento\Framework\DataObject\Factory as ObjectFactory;
+use Magento\Framework\Exception\LocalizedException;
+use Magento\GroupedProduct\Api\Data\GroupedOptionsInterface;
+use Magento\GroupedProduct\Model\Product\Type\Grouped;
+use Magento\Quote\Api\Data as QuoteApi;
+use Magento\Quote\Api\Data\CartItemInterface;
+use Magento\Quote\Model\Quote\Item\CartItemProcessorInterface;
+
+/**
+ * Converts grouped_options to super_group for the grouped product.
+ */
+class CartItemProcessor implements CartItemProcessorInterface
+{
+    private const SUPER_GROUP_CODE = 'super_group';
+
+    /**
+     * @var ObjectFactory
+     */
+    private $objectFactory;
+
+    /**
+     * @var QuoteApi\ProductOptionExtensionFactory
+     */
+    private $productOptionExtensionFactory;
+
+    /**
+     * @var QuoteApi\ProductOptionInterfaceFactory
+     */
+    private $productOptionFactory;
+
+    /**
+     * @var array|null
+     */
+    private $groupedOptions;
+
+    /**
+     * @param ObjectFactory $objectFactory
+     * @param QuoteApi\ProductOptionExtensionFactory $productOptionExtensionFactory
+     * @param QuoteApi\ProductOptionInterfaceFactory $productOptionFactory
+     */
+    public function __construct(
+        ObjectFactory $objectFactory,
+        QuoteApi\ProductOptionExtensionFactory $productOptionExtensionFactory,
+        QuoteApi\ProductOptionInterfaceFactory $productOptionFactory
+    ) {
+        $this->objectFactory = $objectFactory;
+        $this->productOptionExtensionFactory = $productOptionExtensionFactory;
+        $this->productOptionFactory = $productOptionFactory;
+    }
+
+    /**
+     * Converts the grouped_options request data into the same format as native frontend add-to-cart
+     *
+     * @param CartItemInterface $cartItem
+     * @return DataObject|null
+     */
+    public function convertToBuyRequest(CartItemInterface $cartItem): ?DataObject
+    {
+        if ($cartItem->getProductOption()
+            && $cartItem->getProductOption()->getExtensionAttributes()
+            && $cartItem->getProductOption()->getExtensionAttributes()->getGroupedOptions()
+        ) {
+            $groupedOptions = $cartItem->getProductOption()->getExtensionAttributes()->getGroupedOptions();
+            $this->groupedOptions = $groupedOptions;
+
+            return $this->objectFactory->create($this->getConvertedData($groupedOptions));
+        }
+
+        return null;
+    }
+
+    /**
+     * Returns grouped_options converted to super_group data
+     *
+     * @param GroupedOptionsInterface[] $groupedOptions
+     * @return array
+     * @throws LocalizedException
+     */
+    private function getConvertedData(array $groupedOptions): array
+    {
+        $requestData = [];
+        foreach ($groupedOptions as $item) {
+            /** @var GroupedOptionsInterface $item */
+            if ($item->getQty() === null || $item->getId() === null) {
+                throw new LocalizedException(__('Please specify id and qty for grouped options.'));
+            }
+            $requestData[self::SUPER_GROUP_CODE][$item->getId()] = $item->getQty();
+        }
+
+        return $requestData;
+    }
+
+    /**
+     * Option processor
+     *
+     * @param CartItemInterface $cartItem
+     * @return CartItemInterface
+     */
+    public function processOptions(CartItemInterface $cartItem): CartItemInterface
+    {
+        if (empty($this->groupedOptions) || $cartItem->getProductType() !== Grouped::TYPE_CODE) {
+            return $cartItem;
+        }
+
+        $extension = $this->productOptionExtensionFactory->create()
+            ->setGroupedOptions($this->groupedOptions);
+        if (!$cartItem->getProductOption()) {
+            $cartItem->setProductOption($this->productOptionFactory->create());
+        }
+        $cartItem->getProductOption()->setExtensionAttributes($extension);
+
+        return $cartItem;
+    }
+}
diff --git a/app/code/Magento/GroupedProduct/Model/Quote/Item/GroupedOptions.php b/app/code/Magento/GroupedProduct/Model/Quote/Item/GroupedOptions.php
new file mode 100644
index 0000000000000..3b249a7b5d548
--- /dev/null
+++ b/app/code/Magento/GroupedProduct/Model/Quote/Item/GroupedOptions.php
@@ -0,0 +1,79 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\GroupedProduct\Model\Quote\Item;
+
+use Magento\GroupedProduct\Api\Data\GroupedOptionsInterface;
+use Magento\GroupedProduct\Api\Data\GroupedOptionsExtensionInterface;
+
+/**
+ * @inheritDoc
+ */
+class GroupedOptions implements GroupedOptionsInterface
+{
+    /**
+     * @var int|null
+     */
+    private $qty;
+
+    /**
+     * @var int|null
+     */
+    private $id;
+
+    /**
+     * @var GroupedOptionsExtensionInterface|null
+     */
+    private $extensionAttributes;
+
+    /**
+     * @param int|null $id
+     * @param int|null $qty
+     * @param GroupedOptionsExtensionInterface|null $extensionAttributes
+     */
+    public function __construct(
+        ?int $id = null,
+        ?int $qty = null,
+        ?GroupedOptionsExtensionInterface $extensionAttributes = null
+    ) {
+        $this->id = $id;
+        $this->qty = $qty;
+        $this->extensionAttributes = $extensionAttributes;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function getId(): ?int
+    {
+        return $this->id;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function getQty(): ?int
+    {
+        return $this->qty;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function setExtensionAttributes(GroupedOptionsExtensionInterface $extensionAttributes): void
+    {
+        $this->extensionAttributes = $extensionAttributes;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function getExtensionAttributes(): ?GroupedOptionsExtensionInterface
+    {
+        return $this->extensionAttributes;
+    }
+}
diff --git a/app/code/Magento/GroupedProduct/etc/di.xml b/app/code/Magento/GroupedProduct/etc/di.xml
index 924d2d1fc9669..8b58ac08ebd31 100644
--- a/app/code/Magento/GroupedProduct/etc/di.xml
+++ b/app/code/Magento/GroupedProduct/etc/di.xml
@@ -6,6 +6,8 @@
  */
 -->
 <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
+    <preference for="Magento\GroupedProduct\Api\Data\GroupedOptionsInterface" type="Magento\GroupedProduct\Model\Quote\Item\GroupedOptions" />
+
     <type name="Magento\Quote\Model\Quote\Item\RelatedProducts">
         <arguments>
             <argument name="relatedProductTypes" xsi:type="array">
@@ -105,6 +107,13 @@
             </argument>
         </arguments>
     </type>
+    <type name="Magento\Quote\Model\Quote\Item\Repository">
+        <arguments>
+            <argument name="cartItemProcessors" xsi:type="array">
+                <item name="grouped" xsi:type="object">Magento\GroupedProduct\Model\Quote\Item\CartItemProcessor\Proxy</item>
+            </argument>
+        </arguments>
+    </type>
     <type name="Magento\CatalogInventory\Observer\SaveInventoryDataObserver">
         <arguments>
             <argument name="parentItemProcessorPool" xsi:type="array">
diff --git a/app/code/Magento/GroupedProduct/etc/extension_attributes.xml b/app/code/Magento/GroupedProduct/etc/extension_attributes.xml
index 14ff9821025c4..b614a1277de76 100644
--- a/app/code/Magento/GroupedProduct/etc/extension_attributes.xml
+++ b/app/code/Magento/GroupedProduct/etc/extension_attributes.xml
@@ -9,4 +9,8 @@
     <extension_attributes for="Magento\Catalog\Api\Data\ProductLinkInterface">
         <attribute code="qty" type="float" />
     </extension_attributes>
+
+    <extension_attributes for="Magento\Quote\Api\Data\ProductOptionInterface">
+        <attribute code="grouped_options" type="Magento\GroupedProduct\Api\Data\GroupedOptionsInterface[]" />
+    </extension_attributes>
 </config>
diff --git a/dev/tests/api-functional/testsuite/Magento/Quote/Api/CartAddingItemsTest.php b/dev/tests/api-functional/testsuite/Magento/Quote/Api/CartAddingItemsTest.php
index 7900ae45e2f3d..bb2a4e68212cf 100644
--- a/dev/tests/api-functional/testsuite/Magento/Quote/Api/CartAddingItemsTest.php
+++ b/dev/tests/api-functional/testsuite/Magento/Quote/Api/CartAddingItemsTest.php
@@ -7,6 +7,11 @@
 
 namespace Magento\Quote\Api;
 
+use Magento\Catalog\Model\ResourceModel\Product as ProductResource;
+use Magento\Framework\Webapi\Rest\Request;
+use Magento\Integration\Api\CustomerTokenServiceInterface;
+use Magento\TestFramework\Helper\Bootstrap;
+use Magento\TestFramework\ObjectManager;
 use Magento\TestFramework\TestCase\WebapiAbstract;
 
 /**
@@ -15,13 +20,22 @@
 class CartAddingItemsTest extends WebapiAbstract
 {
     /**
-     * @var \Magento\TestFramework\ObjectManager
+     * @var ObjectManager
      */
     protected $objectManager;
 
+    /**
+     * @var ProductResource
+     */
+    private $productResource;
+
+    /**
+     * @inheritDoc
+     */
     protected function setUp(): void
     {
-        $this->objectManager = \Magento\TestFramework\Helper\Bootstrap::getObjectManager();
+        $this->objectManager = Bootstrap::getObjectManager();
+        $this->productResource = $this->objectManager->get(ProductResource::class);
     }
 
     /**
@@ -36,9 +50,9 @@ public function testPriceForCreatingQuoteFromEmptyCart()
         $this->_markTestAsRestOnly();
 
         // Get customer ID token
-        /** @var \Magento\Integration\Api\CustomerTokenServiceInterface $customerTokenService */
+        /** @var CustomerTokenServiceInterface $customerTokenService */
         $customerTokenService = $this->objectManager->create(
-            \Magento\Integration\Api\CustomerTokenServiceInterface::class
+            CustomerTokenServiceInterface::class
         );
         $token = $customerTokenService->createCustomerAccessToken(
             'customer_one_address@test.com',
@@ -49,7 +63,7 @@ public function testPriceForCreatingQuoteFromEmptyCart()
         $serviceInfo = [
             'rest' => [
                 'resourcePath' => '/V1/carts/mine',
-                'httpMethod' => \Magento\Framework\Webapi\Rest\Request::HTTP_METHOD_POST,
+                'httpMethod' => Request::HTTP_METHOD_POST,
                 'token' => $token
             ]
         ];
@@ -58,13 +72,6 @@ public function testPriceForCreatingQuoteFromEmptyCart()
         $this->assertGreaterThan(0, $quoteId);
 
         // Adding item to the cart
-        $serviceInfoForAddingProduct = [
-            'rest' => [
-                'resourcePath' => '/V1/carts/mine/items',
-                'httpMethod' => \Magento\Framework\Webapi\Rest\Request::HTTP_METHOD_POST,
-                'token' => $token
-            ]
-        ];
         $requestData = [
             'cartItem' => [
                 'quote_id' => $quoteId,
@@ -72,7 +79,7 @@ public function testPriceForCreatingQuoteFromEmptyCart()
                 'qty' => 1
             ]
         ];
-        $item = $this->_webApiCall($serviceInfoForAddingProduct, $requestData);
+        $item = $this->_webApiCall($this->getServiceInfoAddToCart($token), $requestData);
         $this->assertNotEmpty($item);
         $this->assertEquals(10, $item['price']);
 
@@ -80,7 +87,7 @@ public function testPriceForCreatingQuoteFromEmptyCart()
         $serviceInfoForGettingPaymentInfo = [
             'rest' => [
                 'resourcePath' => '/V1/carts/mine/payment-information',
-                'httpMethod' => \Magento\Framework\Webapi\Rest\Request::HTTP_METHOD_GET,
+                'httpMethod' => Request::HTTP_METHOD_GET,
                 'token' => $token
             ]
         ];
@@ -92,4 +99,137 @@ public function testPriceForCreatingQuoteFromEmptyCart()
         $quote->load($quoteId);
         $quote->delete();
     }
+
+    /**
+     * Test qty for cart after adding grouped product with custom qty.
+     *
+     * @magentoApiDataFixture Magento/GroupedProduct/_files/product_grouped_with_simple.php
+     * @magentoApiDataFixture Magento/Customer/_files/customer_one_address.php
+     * @return void
+     */
+    public function testAddToCartGroupedCustomQuantity(): void
+    {
+        $this->_markTestAsRestOnly();
+
+        $firstProductId = $this->productResource->getIdBySku('simple_11');
+        $secondProductId = $this->productResource->getIdBySku('simple_22');
+        $qtyData = [$firstProductId => 2, $secondProductId => 4];
+
+        // Get customer ID token
+        /** @var CustomerTokenServiceInterface $customerTokenService */
+        $customerTokenService = $this->objectManager->create(CustomerTokenServiceInterface::class);
+        $token = $customerTokenService->createCustomerAccessToken(
+            'customer_one_address@test.com',
+            'password'
+        );
+
+        // Creating empty cart for registered customer.
+        $serviceInfo = [
+            'rest' => [
+                'resourcePath' => '/V1/carts/mine',
+                'httpMethod' => Request::HTTP_METHOD_POST,
+                'token' => $token
+            ]
+        ];
+
+        $quoteId = $this->_webApiCall($serviceInfo, ['customerId' => 999]); // customerId 999 will get overridden
+        $this->assertGreaterThan(0, $quoteId);
+
+        // Adding item to the cart
+        $productOptionData = [
+            'extension_attributes' => [
+                'grouped_options' => [
+                    ['id' => $firstProductId, 'qty' => $qtyData[$firstProductId]],
+                    ['id' => $secondProductId, 'qty' => $qtyData[$secondProductId]],
+                ]
+            ]
+        ];
+        $requestData = [
+            'cartItem' => [
+                'quote_id' => $quoteId,
+                'sku' => 'grouped',
+                'qty' => 1,
+                'product_option' => $productOptionData
+            ]
+        ];
+        $response = $this->_webApiCall($this->getServiceInfoAddToCart($token), $requestData);
+        $this->assertArrayHasKey('product_option', $response);
+        $this->assertEquals($response['product_option'], $productOptionData);
+
+        /** @var CartRepositoryInterface $cartRepository */
+        $cartRepository = $this->objectManager->get(CartRepositoryInterface::class);
+        $quote = $cartRepository->get($quoteId);
+
+        foreach ($quote->getAllItems() as $item) {
+            $this->assertEquals($qtyData[$item->getProductId()], $item->getQty());
+        }
+    }
+
+    /**
+     * Test adding grouped product when qty for grouped_options not specified.
+     *
+     * @magentoApiDataFixture Magento/GroupedProduct/_files/product_grouped_with_simple.php
+     * @magentoApiDataFixture Magento/Customer/_files/customer_one_address.php
+     * @return void
+     */
+    public function testAddToCartGroupedCustomQuantityNotAllParamsSpecified(): void
+    {
+        $this->_markTestAsRestOnly();
+
+        $productId = $this->productResource->getIdBySku('simple_11');
+
+        // Get customer ID token
+        /** @var CustomerTokenServiceInterface $customerTokenService */
+        $customerTokenService = $this->objectManager->create(CustomerTokenServiceInterface::class);
+        $token = $customerTokenService->createCustomerAccessToken(
+            'customer_one_address@test.com',
+            'password'
+        );
+
+        // Creating empty cart for registered customer.
+        $serviceInfo = [
+            'rest' => ['resourcePath' => '/V1/carts/mine', 'httpMethod' => Request::HTTP_METHOD_POST, 'token' => $token]
+        ];
+
+        $quoteId = $this->_webApiCall($serviceInfo, ['customerId' => 999]); // customerId 999 will get overridden
+        $this->assertGreaterThan(0, $quoteId);
+
+        // Adding item to the cart
+        $requestData = [
+            'cartItem' => [
+                'quote_id' => $quoteId,
+                'sku' => 'grouped',
+                'qty' => 1,
+                'product_option' => [
+                    'extension_attributes' => [
+                        'grouped_options' => [
+                            ['id' => $productId],
+                        ]
+                    ]
+                ]
+            ]
+        ];
+
+        $this->expectException(\Exception::class);
+        $this->expectExceptionMessage('Please specify id and qty for grouped options.');
+
+        $this->_webApiCall($this->getServiceInfoAddToCart($token), $requestData);
+    }
+
+    /**
+     * Returns service info add to cart
+     *
+     * @param string $token
+     * @return array
+     */
+    private function getServiceInfoAddToCart(string $token): array
+    {
+        return [
+            'rest' => [
+                'resourcePath' => '/V1/carts/mine/items',
+                'httpMethod' => Request::HTTP_METHOD_POST,
+                'token' => $token
+            ]
+        ];
+    }
 }