diff --git a/app/code/Magento/QuoteGraphQl/Model/Cart/QuoteAddressFactory.php b/app/code/Magento/QuoteGraphQl/Model/Cart/QuoteAddressFactory.php
index 7f88e9e52f116..895516e783efa 100644
--- a/app/code/Magento/QuoteGraphQl/Model/Cart/QuoteAddressFactory.php
+++ b/app/code/Magento/QuoteGraphQl/Model/Cart/QuoteAddressFactory.php
@@ -16,6 +16,7 @@
 use Magento\Framework\GraphQl\Exception\GraphQlAuthorizationException;
 use Magento\Framework\GraphQl\Exception\GraphQlInputException;
 use Magento\Framework\GraphQl\Exception\GraphQlNoSuchEntityException;
+use Magento\Quote\Api\Data\CartInterface;
 use Magento\Quote\Model\Quote\Address as QuoteAddress;
 use Magento\Quote\Model\Quote\AddressFactory as BaseQuoteAddressFactory;
 
@@ -194,4 +195,21 @@ public function createBasedOnCustomerAddress(int $customerAddressId, int $custom
         }
         return $quoteAddress;
     }
+
+    /**
+     * Create quote address based on the shipping address.
+     *
+     * @param CartInterface $quote
+     * @return QuoteAddress
+     */
+    public function createBasedOnShippingAddress(CartInterface $quote): QuoteAddress
+    {
+        $shippingAddressData = $quote->getShippingAddress()->exportCustomerAddress();
+
+        /** @var QuoteAddress $quoteAddress */
+        $quoteAddress = $this->quoteAddressFactory->create();
+        $quoteAddress->importCustomerAddressData($shippingAddressData);
+
+        return $quoteAddress;
+    }
 }
diff --git a/app/code/Magento/QuoteGraphQl/Model/Cart/SetBillingAddressOnCart.php b/app/code/Magento/QuoteGraphQl/Model/Cart/SetBillingAddressOnCart.php
index 497819cbeec3b..a7d44fa104c27 100644
--- a/app/code/Magento/QuoteGraphQl/Model/Cart/SetBillingAddressOnCart.php
+++ b/app/code/Magento/QuoteGraphQl/Model/Cart/SetBillingAddressOnCart.php
@@ -49,42 +49,46 @@ public function __construct(
      * @param array $billingAddressInput
      * @return void
      * @throws GraphQlAuthorizationException
-     * @throws GraphQlInputException
-     * @throws GraphQlNoSuchEntityException
      */
     public function execute(ContextInterface $context, CartInterface $cart, array $billingAddressInput): void
     {
+        $this->checkForInputExceptions($billingAddressInput);
+
         $customerAddressId = $billingAddressInput['customer_address_id'] ?? null;
         $addressInput = $billingAddressInput['address'] ?? null;
+        $useForShipping = $billingAddressInput['use_for_shipping'] ?? false;
+        $sameAsShipping = $billingAddressInput['same_as_shipping'] ?? false;
 
-        if (!$customerAddressId && !isset($billingAddressInput['address']['save_in_address_book']) && $addressInput) {
+        if (!$customerAddressId && $addressInput && !isset($addressInput['save_in_address_book'])) {
             $addressInput['save_in_address_book'] = true;
         }
 
-        // Need to keep this for BC of `use_for_shipping` field
-        $sameAsShipping = isset($billingAddressInput['use_for_shipping'])
-            ? (bool)$billingAddressInput['use_for_shipping'] : false;
-        $sameAsShipping = isset($billingAddressInput['same_as_shipping'])
-            ? (bool)$billingAddressInput['same_as_shipping'] : $sameAsShipping;
-
-        $this->checkForInputExceptions($billingAddressInput);
-
-        $addresses = $cart->getAllShippingAddresses();
-        if ($sameAsShipping && count($addresses) > 1) {
-            throw new GraphQlInputException(
-                __('Using the "same_as_shipping" option with multishipping is not possible.')
+        if ($sameAsShipping) {
+            $this->validateCanUseShippingForBilling($cart);
+            $billingAddress = $this->quoteAddressFactory->createBasedOnShippingAddress($cart);
+            $useForShipping = false;
+        } elseif ($customerAddressId) {
+            $this->validateCanUseCustomerAddress($context);
+            $billingAddress = $this->quoteAddressFactory->createBasedOnCustomerAddress(
+                (int)$customerAddressId,
+                (int)$context->getUserId()
             );
+        } else {
+            $billingAddress = $this->quoteAddressFactory->createBasedOnInputData($addressInput);
         }
 
-        $billingAddress = $this->createBillingAddress($context, $customerAddressId, $addressInput);
+        if ($useForShipping) {
+            $this->validateCanUseBillingForShipping($cart);
+        }
 
-        $this->assignBillingAddressToCart->execute($cart, $billingAddress, $sameAsShipping);
+        $this->validateBillingAddress($billingAddress);
+        $this->assignBillingAddressToCart->execute($cart, $billingAddress, $useForShipping);
     }
 
     /**
      * Check for the input exceptions
      *
-     * @param array $billingAddressInput
+     * @param array|null $billingAddressInput
      * @throws GraphQlInputException
      */
     private function checkForInputExceptions(
@@ -92,10 +96,11 @@ private function checkForInputExceptions(
     ) {
         $customerAddressId = $billingAddressInput['customer_address_id'] ?? null;
         $addressInput = $billingAddressInput['address'] ?? null;
+        $sameAsShipping = $billingAddressInput['same_as_shipping'] ?? null;
 
-        if (null === $customerAddressId && null === $addressInput) {
+        if (null === $customerAddressId && null === $addressInput && empty($sameAsShipping)) {
             throw new GraphQlInputException(
-                __('The billing address must contain either "customer_address_id" or "address".')
+                __('The billing address must contain either "customer_address_id", "address", or "same_as_shipping".')
             );
         }
 
@@ -107,41 +112,79 @@ private function checkForInputExceptions(
     }
 
     /**
-     * Create billing address
+     * Validate that the quote is capable of using the shipping address as the billing address.
      *
-     * @param ContextInterface $context
-     * @param int|null $customerAddressId
-     * @param array $addressInput
-     * @return Address
-     * @throws GraphQlAuthorizationException
+     * @param CartInterface $quote
      * @throws GraphQlInputException
-     * @throws GraphQlNoSuchEntityException
      */
-    private function createBillingAddress(
-        ContextInterface $context,
-        ?int $customerAddressId,
-        ?array $addressInput
-    ): Address {
-        if (null === $customerAddressId) {
-            $billingAddress = $this->quoteAddressFactory->createBasedOnInputData($addressInput);
-        } else {
-            if (false === $context->getExtensionAttributes()->getIsCustomer()) {
-                throw new GraphQlAuthorizationException(__('The current customer isn\'t authorized.'));
-            }
+    private function validateCanUseShippingForBilling(CartInterface $quote)
+    {
+        $shippingAddresses = $quote->getAllShippingAddresses();
 
-            $billingAddress = $this->quoteAddressFactory->createBasedOnCustomerAddress(
-                (int)$customerAddressId,
-                (int)$context->getUserId()
+        if (count($shippingAddresses) > 1) {
+            throw new GraphQlInputException(
+                __('Could not use the "same_as_shipping" option, because multiple shipping addresses have been set.')
             );
         }
+
+        if (empty($shippingAddresses) || $shippingAddresses[0]->validate() !== true) {
+            throw new GraphQlInputException(
+                __('Could not use the "same_as_shipping" option, because the shipping address has not been set.')
+            );
+        }
+    }
+
+    /**
+     * Validate that the quote is capable of using the billing address as the shipping address.
+     *
+     * @param CartInterface $quote
+     * @throws GraphQlInputException
+     */
+    private function validateCanUseBillingForShipping(CartInterface $quote)
+    {
+        $shippingAddresses = $quote->getAllShippingAddresses();
+
+        if (count($shippingAddresses) > 1) {
+            throw new GraphQlInputException(
+                __('Could not use the "use_for_shipping" option, because multiple shipping addresses have already been set.')
+            );
+        }
+    }
+
+    /**
+     * Validate that the currently logged-in customer is authorized to use a customer address id as the billing address.
+     *
+     * @param ContextInterface $context
+     * @throws GraphQlAuthorizationException
+     */
+    private function validateCanUseCustomerAddress(ContextInterface $context)
+    {
+        if (false === $context->getExtensionAttributes()->getIsCustomer()) {
+            throw new GraphQlAuthorizationException(__('The current customer isn\'t authorized.'));
+        }
+    }
+
+    /**
+     * Validate the billing address to be set on the cart.
+     *
+     * @param Address $billingAddress
+     * @return Address
+     * @throws GraphQlInputException
+     */
+    private function validateBillingAddress(Address $billingAddress)
+    {
         $errors = $billingAddress->validate();
+
         if (true !== $errors) {
-            $e = new GraphQlInputException(__('Billing address errors'));
+            $e = new GraphQlInputException(__('An error occurred while processing the billing address.'));
+
             foreach ($errors as $error) {
                 $e->addError(new GraphQlInputException($error));
             }
+
             throw $e;
         }
+
         return $billingAddress;
     }
 }
diff --git a/app/code/Magento/QuoteGraphQl/etc/schema.graphqls b/app/code/Magento/QuoteGraphQl/etc/schema.graphqls
index 35d7cf082fc94..21688234dc16d 100644
--- a/app/code/Magento/QuoteGraphQl/etc/schema.graphqls
+++ b/app/code/Magento/QuoteGraphQl/etc/schema.graphqls
@@ -104,8 +104,8 @@ input SetBillingAddressOnCartInput {
 input BillingAddressInput {
     customer_address_id: Int
     address: CartAddressInput
-    use_for_shipping: Boolean @doc(description: "Deprecated: use `same_as_shipping` field instead")
-    same_as_shipping: Boolean @doc(description: "Set billing address same as shipping")
+    use_for_shipping: Boolean @doc(description: "Indicates whether to additionally set the shipping address based on the provided billing address")
+    same_as_shipping: Boolean @doc(description: "Indicates whether to set the billing address based on the existing shipping address on the cart")
 }
 
 input CartAddressInput {
diff --git a/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Customer/SetBillingAddressOnCartTest.php b/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Customer/SetBillingAddressOnCartTest.php
index e2e9d809bf8ee..a1ad0763865a8 100644
--- a/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Customer/SetBillingAddressOnCartTest.php
+++ b/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Customer/SetBillingAddressOnCartTest.php
@@ -75,6 +75,8 @@ protected function setUp(): void
     }
 
     /**
+     * Tests setting the billing address on a logged-in customer's cart by providing new address input information.
+     *
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/customer/create_empty_cart.php
@@ -101,7 +103,6 @@ public function testSetNewBillingAddress()
           country_code: "US"
           telephone: "88776655"
          }
-         same_as_shipping: true
       }
     }
   ) {
@@ -120,20 +121,6 @@ public function testSetNewBillingAddress()
         }
         __typename
       }
-      shipping_addresses {
-        firstname
-        lastname
-        company
-        street
-        city
-        postcode
-        telephone
-        country {
-          code
-          label
-        }
-        __typename
-      }
     }
   }
 }
@@ -144,14 +131,11 @@ public function testSetNewBillingAddress()
         $cartResponse = $response['setBillingAddressOnCart']['cart'];
         self::assertArrayHasKey('billing_address', $cartResponse);
         $billingAddressResponse = $cartResponse['billing_address'];
-        self::assertArrayHasKey('shipping_addresses', $cartResponse);
-        $shippingAddressResponse = current($cartResponse['shipping_addresses']);
         $this->assertNewAddressFields($billingAddressResponse);
-        $this->assertNewAddressFields($shippingAddressResponse, 'ShippingCartAddress');
     }
 
     /**
-     * Test case for deprecated `use_for_shipping` param.
+     * Tests that the "use_for_shipping" option sets the provided billing address for shipping as well.
      *
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
@@ -229,6 +213,8 @@ public function testSetNewBillingAddressWithUseForShippingParameter()
     }
 
     /**
+     * Tests setting the billing address on a logged-in customer's cart by providing a saved customer address.
+     *
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/Customer/_files/customer_two_addresses.php
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
@@ -277,6 +263,8 @@ public function testSetBillingAddressFromAddressBook()
     }
 
     /**
+     * Tests that the billing_address output is of type BillingCartAddress.
+     *
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/Customer/_files/customer_two_addresses.php
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
@@ -313,6 +301,8 @@ public function testVerifyBillingAddressType()
     }
 
     /**
+     * Tests that an error occurs when a non-existent customer_address_id is provided.
+     *
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/customer/create_empty_cart.php
@@ -348,6 +338,8 @@ public function testSetNotExistedBillingAddressFromAddressBook()
     }
 
     /**
+     * Tests that an error occurs when both a "customer_address_id" and "address" input are simultaneously provided.
+     *
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/Customer/_files/customer_two_addresses.php
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
@@ -395,6 +387,8 @@ public function testSetNewBillingAddressAndFromAddressBookAtSameTime()
     }
 
     /**
+     * Tests that an error occurs when an address is not provided.
+     *
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/customer/create_empty_cart.php
@@ -406,30 +400,144 @@ public function testSetNewBillingAddressWithoutCustomerAddressIdAndAddress()
 
         $query = <<<QUERY
 mutation {
+  setBillingAddressOnCart(
+    input: {
+      cart_id: "$maskedQuoteId"
+      billing_address: {
+        use_for_shipping: true
+      }
+    }
+  ) {
+    cart {
+      billing_address {
+        city
+      }
+    }
+  }
+}
+QUERY;
+
+        self::expectExceptionMessage(
+           'The billing address must contain either "customer_address_id", "address", or "same_as_shipping".'
+        );
+        $this->graphQlMutation($query, [], '', $this->getHeaderMap());
+    }
+
+    /**
+     * Tests that the "same_as_shipping" option uses the cart's existing shipping address for the billing address.
+     *
+     * Ignores the "customer_address_id" field as well as the "use_for_shipping" option when "same_as_shipping" is true.
+     *
+     * @magentoApiDataFixture Magento/Customer/_files/customer.php
+     * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/customer/create_empty_cart.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/add_simple_product.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/set_new_shipping_address.php
+     */
+    public function testSetBillingAddressWithSameAsShipping()
+    {
+        $maskedQuoteId = $this->getMaskedQuoteIdByReservedOrderId->execute('test_quote');
+
+        $query = <<<QUERY
+mutation {
   setBillingAddressOnCart(
     input: {
       cart_id: "$maskedQuoteId"
       billing_address: {
         same_as_shipping: true
+        use_for_shipping: true
+        customer_address_id: 1
       }
     }
   ) {
     cart {
       billing_address {
+        firstname
+        lastname
+        company
+        street
+        city
+        postcode
+        telephone
+        country {
+          code
+          label
+        }
+        __typename
+      }
+      shipping_addresses {
+        firstname
+        lastname
+        company
+        street
         city
+        postcode
+        telephone
+        country {
+          code
+          label
+        }
+        __typename
       }
     }
   }
 }
 QUERY;
+        $response = $this->graphQlMutation($query, [], '', $this->getHeaderMap());
 
+        self::assertArrayHasKey('cart', $response['setBillingAddressOnCart']);
+        $cartResponse = $response['setBillingAddressOnCart']['cart'];
+
+        // Assert billing address has been set according to the cart's shipping address
+        self::assertArrayHasKey('billing_address', $cartResponse);
+        $billingAddressResponse = $cartResponse['billing_address'];
+        $this->assertNewAddressFieldsFromShippingAddress($billingAddressResponse);
+
+        // Assert the shipping address is unchanged
+        self::assertArrayHasKey('shipping_addresses', $cartResponse);
+        $shippingAddressResponse = current($cartResponse['shipping_addresses']);
+        $this->assertNewAddressFieldsFromShippingAddress($shippingAddressResponse, 'ShippingCartAddress');
+    }
+
+    /**
+     * Tests that the "same_as_shipping" option cannot be used when a shipping address has not been set on the cart.
+     *
+     * @magentoApiDataFixture Magento/Customer/_files/customer.php
+     * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/customer/create_empty_cart.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/add_simple_product.php
+     */
+    public function testSetBillingAddressWithSameAsShippingWithoutShippingAddressOnCart()
+    {
+        $maskedQuoteId = $this->getMaskedQuoteIdByReservedOrderId->execute('test_quote');
+
+        $query = <<<QUERY
+mutation {
+  setBillingAddressOnCart(
+    input: {
+      cart_id: "$maskedQuoteId"
+      billing_address: {
+        same_as_shipping: true
+      }
+    }
+  ) {
+    cart {
+      billing_address {
+        city
+      }
+    }
+  }
+}
+QUERY;
         self::expectExceptionMessage(
-            'The billing address must contain either "customer_address_id" or "address".'
+            'Could not use the "same_as_shipping" option, because the shipping address has not been set.'
         );
         $this->graphQlMutation($query, [], '', $this->getHeaderMap());
     }
 
     /**
+     * Tests that the "same_as_shipping" option cannot be used when multi-shipping is applied.
+     *
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/customer/create_empty_cart.php
@@ -446,17 +554,6 @@ public function testSetNewBillingAddressWithSameAsShippingAndMultishipping()
     input: {
       cart_id: "$maskedQuoteId"
       billing_address: {
-        address: {
-          firstname: "test firstname"
-          lastname: "test lastname"
-          company: "test company"
-          street: ["test street 1", "test street 2"]
-          city: "test city"
-          region: "AZ"
-          postcode: "887766"
-          country_code: "US"
-          telephone: "88776655"
-        }
         same_as_shipping: true
       }
     }
@@ -471,12 +568,14 @@ public function testSetNewBillingAddressWithSameAsShippingAndMultishipping()
 QUERY;
 
         self::expectExceptionMessage(
-            'Using the "same_as_shipping" option with multishipping is not possible.'
+            'Could not use the "same_as_shipping" option, because multiple shipping addresses have been set.'
         );
         $this->graphQlMutation($query, [], '', $this->getHeaderMap());
     }
 
     /**
+     * Tests that a logged-in customer cannot set the billing address on a guest cart.
+     *
      * _security
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/Customer/_files/customer_address.php
@@ -514,6 +613,8 @@ public function testSetBillingAddressToGuestCart()
     }
 
     /**
+     * Tests that a logged-in customer cannot set the billing address on a cart they do not own.
+     *
      * _security
      * @magentoApiDataFixture Magento/Customer/_files/three_customers.php
      * @magentoApiDataFixture Magento/Customer/_files/customer_address.php
@@ -549,6 +650,8 @@ public function testSetBillingAddressToAnotherCustomerCart()
     }
 
     /**
+     * Tests that a logged-in customer cannot use a saved customer address that is not their own.
+     *
      * _security
      * @magentoApiDataFixture Magento/Customer/_files/three_customers.php
      * @magentoApiDataFixture Magento/Customer/_files/customer_address.php
@@ -584,6 +687,8 @@ public function testSetBillingAddressIfCustomerIsNotOwnerOfAddress()
     }
 
     /**
+     * Tests that an error occurs when attempting to set the billing address on a cart that does not exist.
+     *
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/Customer/_files/customer_address.php
      */
@@ -615,6 +720,8 @@ public function testSetBillingAddressOnNonExistentCart()
     }
 
     /**
+     * Test that an error occurs when ommitting required fields in the address input.
+     *
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/customer/create_empty_cart.php
@@ -727,6 +834,8 @@ public function dataProviderSetWithoutRequiredParameters(): array
     }
 
     /**
+     * Tests that an error occurs when the street information exceeds the maximum number of allowed lines.
+     *
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/customer/create_empty_cart.php
@@ -769,6 +878,8 @@ public function testSetNewBillingAddressWithRedundantStreetLine()
     }
 
     /**
+     * Tests that setting a new billing address succeeds with a lower case "country_code".
+     *
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/customer/create_empty_cart.php
@@ -827,6 +938,9 @@ public function testSetBillingAddressWithLowerCaseCountry()
     }
 
     /**
+     * Tests that setting the "save_in_address_book" option to true adds the newly set billing address to the
+     * customer's address book.
+     *
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/customer/create_empty_cart.php
@@ -894,6 +1008,9 @@ public function testSetNewBillingAddressWithSaveInAddressBook()
     }
 
     /**
+     * Tests that setting the "save_in_address_book" option to false does not add the newly set billing address
+     * to the customer's address book.
+     *
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/customer/create_empty_cart.php
@@ -961,6 +1078,8 @@ public function testSetNewBillingAddressWithNotSaveInAddressBook()
     }
 
     /**
+     * Tests that an error occurs when providing invalid address input.
+     *
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/customer/create_empty_cart.php
@@ -1445,8 +1564,10 @@ public function testSetBillingAddressOnCartWithCorrectRegionId()
      * @param array $addressResponse
      * @param string $addressType
      */
-    private function assertNewAddressFields(array $addressResponse, string $addressType = 'BillingCartAddress'): void
-    {
+    private function assertNewAddressFields(
+        array $addressResponse,
+        string $addressType = 'BillingCartAddress'
+    ): void {
         $assertionMap = [
             ['response_field' => 'firstname', 'expected_value' => 'test firstname'],
             ['response_field' => 'lastname', 'expected_value' => 'test lastname'],
@@ -1462,6 +1583,33 @@ private function assertNewAddressFields(array $addressResponse, string $addressT
         $this->assertResponseFields($addressResponse, $assertionMap);
     }
 
+    /**
+     * Verify that the fields for the specified quote address match the shipping address from the fixture.
+     *
+     * Useful for verifying scenarios with the "same_as_shipping" option.
+     *
+     * @param array $addressResponse
+     * @param string $addressType
+     */
+    private function assertNewAddressFieldsFromShippingAddress(
+        array $addressResponse,
+        string $addressType = 'BillingCartAddress'
+    ): void {
+        $assertionMap = [
+            ['response_field' => 'firstname', 'expected_value' => 'John'],
+            ['response_field' => 'lastname', 'expected_value' => 'Smith'],
+            ['response_field' => 'company', 'expected_value' => 'CompanyName'],
+            ['response_field' => 'street', 'expected_value' => [0 => 'Green str, 67']],
+            ['response_field' => 'city', 'expected_value' => 'CityM'],
+            ['response_field' => 'postcode', 'expected_value' => '75477'],
+            ['response_field' => 'telephone', 'expected_value' => 3468676],
+            ['response_field' => 'country', 'expected_value' => ['code' => 'US', 'label' => 'US']],
+            ['response_field' => '__typename', 'expected_value' => $addressType]
+        ];
+
+        $this->assertResponseFields($addressResponse, $assertionMap);
+    }
+
     /**
      * Verify the all the whitelisted fields for a Address Object
      *
@@ -1566,7 +1714,7 @@ public function testSetBillingAddressAndPlaceOrder()
     input: {
       cart_id: "$maskedQuoteId"
       billing_address: {
-        same_as_shipping: true
+         use_for_shipping: true
          address: {
           firstname: "test firstname"
             lastname: "test lastname"
@@ -1649,7 +1797,7 @@ public function testSetBillingAddressWithDefaultValueOfSaveInAddressBookAndPlace
     input: {
       cart_id: "$maskedQuoteId"
       billing_address: {
-        same_as_shipping: true
+         use_for_shipping: true
          address: {
           firstname: "test firstname"
             lastname: "test lastname"
diff --git a/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Guest/SetBillingAddressOnCartTest.php b/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Guest/SetBillingAddressOnCartTest.php
index d6780b311aff6..764fc3f573a2b 100644
--- a/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Guest/SetBillingAddressOnCartTest.php
+++ b/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Guest/SetBillingAddressOnCartTest.php
@@ -28,6 +28,8 @@ protected function setUp(): void
     }
 
     /**
+     * Tests setting the billing address on the cart by providing address input information.
+     *
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/guest/create_empty_cart.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/add_simple_product.php
@@ -53,7 +55,6 @@ public function testSetNewBillingAddress()
           country_code: "US"
           telephone: "88776655"
          }
-         same_as_shipping: true
       }
     }
   ) {
@@ -72,20 +73,6 @@ public function testSetNewBillingAddress()
         }
         __typename
       }
-      shipping_addresses {
-        firstname
-        lastname
-        company
-        street
-        city
-        postcode
-        telephone
-        country {
-          code
-          label
-        }
-        __typename
-      }
     }
   }
 }
@@ -97,20 +84,17 @@ public function testSetNewBillingAddress()
         self::assertArrayHasKey('billing_address', $cartResponse);
         $billingAddressResponse = $cartResponse['billing_address'];
         $this->assertNewAddressFields($billingAddressResponse);
-        self::assertArrayHasKey('shipping_addresses', $cartResponse);
-        $shippingAddressResponse = current($cartResponse['shipping_addresses']);
         $this->assertNewAddressFields($billingAddressResponse);
-        $this->assertNewAddressFields($shippingAddressResponse, 'ShippingCartAddress');
     }
 
     /**
-     * Test case for deprecated `use_for_shipping` param.
+     * Tests that the "use_for_shipping" option sets the provided billing address for shipping as well.
      *
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/guest/create_empty_cart.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/add_simple_product.php
      */
-    public function testSetNewBillingAddressWithUseForShippingParameter()
+    public function testSetNewBillingAddressWithUseForShipping()
     {
         $maskedQuoteId = $this->getMaskedQuoteIdByReservedOrderId->execute('test_quote');
 
@@ -181,6 +165,8 @@ public function testSetNewBillingAddressWithUseForShippingParameter()
     }
 
     /**
+     * Tests that a guest cannot set the billing address on a logged-in customer's cart.
+     *
      * _security
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/customer/create_empty_cart.php
@@ -225,6 +211,8 @@ public function testSetBillingAddressToCustomerCart()
     }
 
     /**
+     * Tests that a guest cannot use a logged-in customer's saved address as the billing address on their own cart.
+     *
      * _security
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/guest/create_empty_cart.php
@@ -260,6 +248,7 @@ public function testSetBillingAddressFromAddressBook()
     }
 
     /**
+     * Tests that a guest cannot set the billing address on a cart that does not exist.
      */
     public function testSetBillingAddressOnNonExistentCart()
     {
@@ -299,11 +288,13 @@ public function testSetBillingAddressOnNonExistentCart()
     }
 
     /**
+     * Tests that an error is thrown when the street information exceeds the maximum number of allowed lines.
+     *
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/guest/create_empty_cart.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/add_simple_product.php
      */
-    public function testSetNewBillingAddressWithoutCustomerAddressIdAndAddress()
+    public function testSetBillingAddressRedundantStreetLine()
     {
         $maskedQuoteId = $this->getMaskedQuoteIdByReservedOrderId->execute('test_quote');
 
@@ -313,32 +304,41 @@ public function testSetNewBillingAddressWithoutCustomerAddressIdAndAddress()
     input: {
       cart_id: "$maskedQuoteId"
       billing_address: {
-        same_as_shipping: true
+        address: {
+          firstname: "test firstname"
+          lastname: "test lastname"
+          company: "test company"
+          street: ["test street 1", "test street 2", "test street 3"]
+          city: "test city"
+          region: "AL"
+          postcode: "887766"
+          country_code: "US"
+          telephone: "88776655"
+        }
       }
     }
   ) {
     cart {
       billing_address {
-        city
+        firstname
       }
     }
   }
 }
 QUERY;
 
-        self::expectExceptionMessage(
-            'The billing address must contain either "customer_address_id" or "address".'
-        );
+        self::expectExceptionMessage('"Street Address" cannot contain more than 2 lines.');
         $this->graphQlMutation($query);
     }
 
     /**
+     * Tests that setting a new billing address succeeds with a lower case "country_code".
+     *
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/guest/create_empty_cart.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/add_simple_product.php
-     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/set_multishipping_with_two_shipping_addresses.php
      */
-    public function testSetNewBillingAddressWithSameAsShippingAndMultishipping()
+    public function testSetBillingAddressWithLowerCaseCountry()
     {
         $maskedQuoteId = $this->getMaskedQuoteIdByReservedOrderId->execute('test_quote');
 
@@ -356,34 +356,48 @@ public function testSetNewBillingAddressWithSameAsShippingAndMultishipping()
           city: "test city"
           region: "AL"
           postcode: "887766"
-          country_code: "US"
+          country_code: "us"
           telephone: "88776655"
         }
-        same_as_shipping: true
       }
     }
   ) {
     cart {
       billing_address {
+        firstname
+        lastname
+        company
+        street
         city
+        postcode
+        telephone
+        country {
+          code
+          label
+        }
+        __typename
       }
     }
   }
 }
 QUERY;
+        $response = $this->graphQlMutation($query);
 
-        self::expectExceptionMessage(
-            'Using the "same_as_shipping" option with multishipping is not possible.'
-        );
-        $this->graphQlMutation($query);
+        self::assertArrayHasKey('cart', $response['setBillingAddressOnCart']);
+        $cartResponse = $response['setBillingAddressOnCart']['cart'];
+        self::assertArrayHasKey('billing_address', $cartResponse);
+        $billingAddressResponse = $cartResponse['billing_address'];
+        $this->assertNewAddressFields($billingAddressResponse);
     }
 
     /**
+     * Tests that an error is thrown when a billing address is not provided.
+     *
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/guest/create_empty_cart.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/add_simple_product.php
      */
-    public function testSetNewBillingAddressRedundantStreetLine()
+    public function testSetBillingAddressWithoutProvidingAddress()
     {
         $maskedQuoteId = $this->getMaskedQuoteIdByReservedOrderId->execute('test_quote');
 
@@ -393,39 +407,36 @@ public function testSetNewBillingAddressRedundantStreetLine()
     input: {
       cart_id: "$maskedQuoteId"
       billing_address: {
-        address: {
-          firstname: "test firstname"
-          lastname: "test lastname"
-          company: "test company"
-          street: ["test street 1", "test street 2", "test street 3"]
-          city: "test city"
-          region: "AL"
-          postcode: "887766"
-          country_code: "US"
-          telephone: "88776655"
-        }
+        use_for_shipping: true
       }
     }
   ) {
     cart {
       billing_address {
-        firstname
+        city
       }
     }
   }
 }
 QUERY;
 
-        self::expectExceptionMessage('"Street Address" cannot contain more than 2 lines.');
+        self::expectExceptionMessage(
+            'The billing address must contain either "customer_address_id", "address", or "same_as_shipping".'
+        );
         $this->graphQlMutation($query);
     }
 
     /**
+     * Tests that the "same_as_shipping" option uses the cart's existing shipping address for the billing address.
+     *
+     * Ignores any address input as well as the "use_for_shipping" option when "same_as_shipping" is true.
+     *
      * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/guest/create_empty_cart.php
      * @magentoApiDataFixture Magento/GraphQl/Quote/_files/add_simple_product.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/set_new_shipping_address.php
      */
-    public function testSetBillingAddressWithLowerCaseCountry()
+    public function testSetBillingAddressWithSameAsShipping()
     {
         $maskedQuoteId = $this->getMaskedQuoteIdByReservedOrderId->execute('test_quote');
 
@@ -435,6 +446,8 @@ public function testSetBillingAddressWithLowerCaseCountry()
     input: {
       cart_id: "$maskedQuoteId"
       billing_address: {
+        same_as_shipping: true
+        use_for_shipping: true
         address: {
           firstname: "test firstname"
           lastname: "test lastname"
@@ -443,7 +456,7 @@ public function testSetBillingAddressWithLowerCaseCountry()
           city: "test city"
           region: "AL"
           postcode: "887766"
-          country_code: "us"
+          country_code: "US"
           telephone: "88776655"
         }
       }
@@ -464,6 +477,20 @@ public function testSetBillingAddressWithLowerCaseCountry()
         }
         __typename
       }
+      shipping_addresses {
+        firstname
+        lastname
+        company
+        street
+        city
+        postcode
+        telephone
+        country {
+          code
+          label
+        }
+        __typename
+      }
     }
   }
 }
@@ -472,19 +499,100 @@ public function testSetBillingAddressWithLowerCaseCountry()
 
         self::assertArrayHasKey('cart', $response['setBillingAddressOnCart']);
         $cartResponse = $response['setBillingAddressOnCart']['cart'];
+
+        // Assert billing address has been set according to the cart's shipping address
         self::assertArrayHasKey('billing_address', $cartResponse);
         $billingAddressResponse = $cartResponse['billing_address'];
-        $this->assertNewAddressFields($billingAddressResponse);
+        $this->assertNewAddressFieldsFromShippingAddress($billingAddressResponse);
+
+        // Assert the shipping address is unchanged
+        self::assertArrayHasKey('shipping_addresses', $cartResponse);
+        $shippingAddressResponse = current($cartResponse['shipping_addresses']);
+        $this->assertNewAddressFieldsFromShippingAddress($shippingAddressResponse, 'ShippingCartAddress');
     }
 
     /**
-     * Verify the all the whitelisted fields for a New Address Object
+     * Tests that the "same_as_shipping" option cannot be used when a shipping address has not been set on the cart.
+     *
+     * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/guest/create_empty_cart.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/add_simple_product.php
+     */
+    public function testSetBillingAddressWithSameAsShippingWithoutShippingAddressOnCart()
+    {
+        $maskedQuoteId = $this->getMaskedQuoteIdByReservedOrderId->execute('test_quote');
+
+        $query = <<<QUERY
+mutation {
+  setBillingAddressOnCart(
+    input: {
+      cart_id: "$maskedQuoteId"
+      billing_address: {
+        same_as_shipping: true
+      }
+    }
+  ) {
+    cart {
+      billing_address {
+        city
+      }
+    }
+  }
+}
+QUERY;
+        self::expectExceptionMessage(
+            'Could not use the "same_as_shipping" option, because the shipping address has not been set.'
+        );
+        $this->graphQlMutation($query);
+    }
+
+    /**
+     * Tests that the "same_as_shipping" option cannot be used when multi-shipping is applied.
+     *
+     * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/guest/create_empty_cart.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/add_simple_product.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/set_multishipping_with_two_shipping_addresses.php
+     */
+    public function testSetBillingAddressWithSameAsShippingAndMultishipping()
+    {
+        $maskedQuoteId = $this->getMaskedQuoteIdByReservedOrderId->execute('test_quote');
+
+        $query = <<<QUERY
+mutation {
+  setBillingAddressOnCart(
+    input: {
+      cart_id: "$maskedQuoteId"
+      billing_address: {
+        same_as_shipping: true
+      }
+    }
+  ) {
+    cart {
+      billing_address {
+        city
+      }
+    }
+  }
+}
+QUERY;
+
+        self::expectExceptionMessage(
+            'Could not use the "same_as_shipping" option, because multiple shipping addresses have been set.'
+        );
+        $this->graphQlMutation($query);
+    }
+
+    /**
+     * Verify all the whitelisted fields for a New Address Object
      *
      * @param array $addressResponse
      * @param string $addressType
      */
-    private function assertNewAddressFields(array $addressResponse, string $addressType = 'BillingCartAddress'): void
-    {
+    private function assertNewAddressFields(
+        array $addressResponse,
+        string $addressType = 'BillingCartAddress'
+    ): void {
         $assertionMap = [
             ['response_field' => 'firstname', 'expected_value' => 'test firstname'],
             ['response_field' => 'lastname', 'expected_value' => 'test lastname'],
@@ -499,4 +607,31 @@ private function assertNewAddressFields(array $addressResponse, string $addressT
 
         $this->assertResponseFields($addressResponse, $assertionMap);
     }
+
+    /**
+     * Verify that the fields for the specified quote address match the shipping address from the fixture.
+     *
+     * Useful for verifying scenarios with the "same_as_shipping" option.
+     *
+     * @param array $addressResponse
+     * @param string $addressType
+     */
+    private function assertNewAddressFieldsFromShippingAddress(
+        array $addressResponse,
+        string $addressType = 'BillingCartAddress'
+    ): void {
+        $assertionMap = [
+            ['response_field' => 'firstname', 'expected_value' => 'John'],
+            ['response_field' => 'lastname', 'expected_value' => 'Smith'],
+            ['response_field' => 'company', 'expected_value' => 'CompanyName'],
+            ['response_field' => 'street', 'expected_value' => [0 => 'Green str, 67']],
+            ['response_field' => 'city', 'expected_value' => 'CityM'],
+            ['response_field' => 'postcode', 'expected_value' => '75477'],
+            ['response_field' => 'telephone', 'expected_value' => 3468676],
+            ['response_field' => 'country', 'expected_value' => ['code' => 'US', 'label' => 'US']],
+            ['response_field' => '__typename', 'expected_value' => $addressType]
+        ];
+
+        $this->assertResponseFields($addressResponse, $assertionMap);
+    }
 }