diff --git a/src/controllers/CartController.php b/src/controllers/CartController.php
index 43aaf18298..9bdc7bcfea 100644
--- a/src/controllers/CartController.php
+++ b/src/controllers/CartController.php
@@ -399,6 +399,24 @@ public function actionLoadCart(): ?Response
             return $this->request->getIsGet() ? $this->redirect($redirect) : null;
         }
 
+        // Check if the cart belongs to a credentialed user
+        $cartCustomer = $cart->getCustomer();
+        $currentUser = Craft::$app->getUser()->getIdentity();
+
+        if ($cartCustomer && $cartCustomer->getIsCredentialed()) {
+            // Only allow loading if the current user is the cart customer
+            if (!$currentUser || $currentUser->id !== $cartCustomer->id) {
+                $error = Craft::t('commerce', 'Unable to retrieve cart.');
+
+                if ($this->request->getAcceptsJson()) {
+                    return $this->asFailure($error);
+                }
+
+                $this->setFailFlash($error);
+                return $this->request->getIsGet() ? $this->redirect($redirect) : null;
+            }
+        }
+
         // If we have a cart, use the site for that cart for the URL redirect.
         $redirect = UrlHelper::siteUrl(path: $loadCartRedirectUrl, siteId: $cart->orderSiteId);
 
diff --git a/tests/unit/controllers/LoadCartTest.php b/tests/unit/controllers/LoadCartTest.php
new file mode 100644
index 0000000000..4e65ae24d3
--- /dev/null
+++ b/tests/unit/controllers/LoadCartTest.php
@@ -0,0 +1,288 @@
+<?php
+/**
+ * @link https://craftcms.com/
+ * @copyright Copyright (c) Pixel & Tonic, Inc.
+ * @license https://craftcms.github.io/license/
+ */
+
+namespace craftcommercetests\unit\controllers;
+
+use Codeception\Test\Unit;
+use Craft;
+use craft\commerce\controllers\CartController;
+use craft\commerce\elements\Order;
+use craft\commerce\Plugin;
+use craft\elements\User;
+use craft\web\Request;
+use craftcommercetests\fixtures\CustomerFixture;
+use Throwable;
+use UnitTester;
+use yii\base\InvalidRouteException;
+use yii\web\Response;
+
+/**
+ * LoadCartTest
+ *
+ * Tests for loading carts via the cart controller.
+ *
+ * Test scenarios:
+ * - CART-A: Cart belonging to a customer with an email for a non-credentialed user (inactive user)
+ * - CART-B: Cart with no email (no customer)
+ * - CART-C: Cart with an email (customer) of a credentialed user
+ *
+ * @author Pixel & Tonic, Inc. <support@pixelandtonic.com>
+ * @since 5.x
+ */
+class LoadCartTest extends Unit
+{
+    /**
+     * @var UnitTester
+     */
+    protected UnitTester $tester;
+
+    /**
+     * @var CartController
+     */
+    protected CartController $cartController;
+
+    /**
+     * @var Request
+     */
+    protected Request $request;
+
+    /**
+     * @var Order|null CART-A: Cart for inactive (non-credentialed) user
+     */
+    private ?Order $_cartA = null;
+
+    /**
+     * @var Order|null CART-B: Cart with no customer
+     */
+    private ?Order $_cartB = null;
+
+    /**
+     * @var Order|null CART-C: Cart for credentialed user
+     */
+    private ?Order $_cartC = null;
+
+    /**
+     * @return array
+     */
+    public function _fixtures(): array
+    {
+        return [
+            'customer' => [
+                'class' => CustomerFixture::class,
+            ],
+        ];
+    }
+
+    /**
+     * @inheritDoc
+     */
+    protected function _before(): void
+    {
+        parent::_before();
+
+        $this->cartController = new CartController('cart', Plugin::getInstance());
+        $this->request = Craft::$app->getRequest();
+        $this->request->enableCsrfValidation = false;
+
+        // Create the test carts
+        $this->_createTestCarts();
+    }
+
+    /**
+     * @inheritDoc
+     */
+    protected function _after(): void
+    {
+        parent::_after();
+
+        // Clean up test carts
+        $this->_cleanupTestCarts();
+
+        // Forget any cart in session
+        Plugin::getInstance()->getCarts()->forgetCart();
+    }
+
+    /**
+     * Create test carts for the scenarios.
+     */
+    private function _createTestCarts(): void
+    {
+        $customerFixture = $this->tester->grabFixture('customer');
+        $store = Plugin::getInstance()->getStores()->getCurrentStore();
+
+        // CART-A: Cart for inactive (non-credentialed) user
+        /** @var User $inactiveUser */
+        $inactiveUser = $customerFixture->getElement('inactive-user');
+        $this->_cartA = new Order();
+        $this->_cartA->number = Plugin::getInstance()->getCarts()->generateCartNumber();
+        $this->_cartA->storeId = $store->id;
+        $this->_cartA->setCustomer($inactiveUser);
+        Craft::$app->getElements()->saveElement($this->_cartA, false);
+
+        // CART-B: Cart with no customer (no email)
+        $this->_cartB = new Order();
+        $this->_cartB->number = Plugin::getInstance()->getCarts()->generateCartNumber();
+        $this->_cartB->storeId = $store->id;
+        Craft::$app->getElements()->saveElement($this->_cartB, false);
+
+        // CART-C: Cart for credentialed user
+        /** @var User $credentialedUser */
+        $credentialedUser = $customerFixture->getElement('credentialed-user');
+        $this->_cartC = new Order();
+        $this->_cartC->number = Plugin::getInstance()->getCarts()->generateCartNumber();
+        $this->_cartC->storeId = $store->id;
+        $this->_cartC->setCustomer($credentialedUser);
+        Craft::$app->getElements()->saveElement($this->_cartC, false);
+    }
+
+    /**
+     * Clean up test carts.
+     */
+    private function _cleanupTestCarts(): void
+    {
+        foreach ([$this->_cartA, $this->_cartB, $this->_cartC] as $cart) {
+            if ($cart && $cart->id) {
+                try {
+                    Craft::$app->getElements()->deleteElement($cart, true);
+                } catch (Throwable) {
+                    // Ignore cleanup errors
+                }
+            }
+        }
+
+        $this->_cartA = null;
+        $this->_cartB = null;
+        $this->_cartC = null;
+    }
+
+    /**
+     * Test that an anonymous user can load a cart with no customer (CART-B).
+     *
+     * @throws InvalidRouteException
+     */
+    public function testAnonymousUserCanLoadCartWithNoCustomer(): void
+    {
+        Plugin::getInstance()->getCarts()->forgetCart();
+
+        $this->request->headers->set('Accept', 'application/json');
+        $this->request->setQueryParams([
+            'number' => $this->_cartB->number,
+        ]);
+
+        $response = $this->cartController->runAction('load-cart');
+
+        self::assertInstanceOf(Response::class, $response);
+        self::assertTrue($response->getIsSuccessful(), 'Anonymous user should be able to load cart with no customer');
+
+        // Verify the cart exists and can be retrieved by number
+        $loadedCart = Order::find()->number($this->_cartB->number)->isCompleted(false)->one();
+        self::assertNotNull($loadedCart, 'CART-B should exist and be retrievable');
+        self::assertSame($this->_cartB->id, $loadedCart->id, 'Loaded cart ID should match CART-B');
+        self::assertNull($loadedCart->getEmail(), 'CART-B should have no email');
+    }
+
+    /**
+     * Test that an anonymous user can load a cart belonging to a non-credentialed (inactive) user (CART-A).
+     *
+     * @throws InvalidRouteException
+     */
+    public function testAnonymousUserCanLoadCartWithInactiveUser(): void
+    {
+        Plugin::getInstance()->getCarts()->forgetCart();
+
+        $this->request->headers->set('Accept', 'application/json');
+        $this->request->setQueryParams([
+            'number' => $this->_cartA->number,
+        ]);
+
+        $response = $this->cartController->runAction('load-cart');
+
+        self::assertInstanceOf(Response::class, $response);
+        self::assertTrue($response->getIsSuccessful(), 'Anonymous user should be able to load cart belonging to inactive user');
+
+        // Verify the cart exists and has the inactive user's email
+        $loadedCart = Order::find()->number($this->_cartA->number)->isCompleted(false)->one();
+        self::assertNotNull($loadedCart, 'CART-A should exist and be retrievable');
+        self::assertSame($this->_cartA->id, $loadedCart->id, 'Loaded cart ID should match CART-A');
+        self::assertSame($this->_cartA->getEmail(), $loadedCart->getEmail(), 'Loaded cart should have inactive user email');
+
+        // Verify the cart customer is NOT credentialed (is inactive)
+        $cartCustomer = $loadedCart->getCustomer();
+        self::assertNotNull($cartCustomer, 'CART-A should have a customer');
+        self::assertFalse($cartCustomer->getIsCredentialed(), 'CART-A customer should not be credentialed');
+    }
+
+    /**
+     * Test that an anonymous user cannot load a cart belonging to a credentialed user (CART-C).
+     *
+     * @throws InvalidRouteException
+     */
+    public function testAnonymousUserCannotLoadCartWithCredentialedUser(): void
+    {
+        Plugin::getInstance()->getCarts()->forgetCart();
+
+        $this->request->headers->set('Accept', 'application/json');
+        $this->request->setQueryParams([
+            'number' => $this->_cartC->number,
+        ]);
+
+        $response = $this->cartController->runAction('load-cart');
+
+        self::assertInstanceOf(Response::class, $response);
+        self::assertFalse($response->getIsSuccessful(), 'Anonymous user should not be able to load cart belonging to credentialed user');
+
+        // Verify CART-C still exists and belongs to the credentialed user (wasn't modified)
+        $cartC = Order::find()->number($this->_cartC->number)->isCompleted(false)->one();
+        self::assertNotNull($cartC, 'CART-C should still exist');
+        self::assertSame($this->_cartC->id, $cartC->id, 'CART-C should be unchanged');
+
+        // Verify the cart customer IS credentialed
+        $cartCustomer = $cartC->getCustomer();
+        self::assertNotNull($cartCustomer, 'CART-C should have a customer');
+        self::assertTrue($cartCustomer->getIsCredentialed(), 'CART-C customer should be credentialed');
+    }
+
+    /**
+     * Test that a credentialed user can load their own cart (CART-C).
+     *
+     * @throws InvalidRouteException
+     */
+    public function testCredentialedUserCanLoadOwnCart(): void
+    {
+        // Log in as the credentialed user
+        $customerFixture = $this->tester->grabFixture('customer');
+        /** @var User $credentialedUser */
+        $credentialedUser = $customerFixture->getElement('credentialed-user');
+        Craft::$app->getUser()->setIdentity(
+            Craft::$app->getUsers()->getUserById($credentialedUser->id)
+        );
+
+        Plugin::getInstance()->getCarts()->forgetCart();
+
+        $this->request->headers->set('Accept', 'application/json');
+        $this->request->setQueryParams([
+            'number' => $this->_cartC->number,
+        ]);
+
+        $response = $this->cartController->runAction('load-cart');
+
+        self::assertInstanceOf(Response::class, $response);
+        self::assertTrue($response->getIsSuccessful(), 'Credentialed user should be able to load their own cart');
+
+        // Verify the cart exists and belongs to the credentialed user
+        $loadedCart = Order::find()->number($this->_cartC->number)->isCompleted(false)->one();
+        self::assertNotNull($loadedCart, 'CART-C should exist and be retrievable');
+        self::assertSame($this->_cartC->id, $loadedCart->id, 'Loaded cart ID should match CART-C');
+        self::assertSame($credentialedUser->id, $loadedCart->getCustomerId(), 'Loaded cart should belong to the credentialed user');
+        self::assertSame($credentialedUser->email, $loadedCart->getEmail(), 'Loaded cart email should match credentialed user email');
+
+        // Verify the cart customer IS credentialed
+        $cartCustomer = $loadedCart->getCustomer();
+        self::assertNotNull($cartCustomer, 'CART-C should have a customer');
+        self::assertTrue($cartCustomer->getIsCredentialed(), 'CART-C customer should be credentialed');
+    }
+}