diff --git a/.changeset/cart-get-respects-cartid.md b/.changeset/cart-get-respects-cartid.md
new file mode 100644
index 0000000000..7e425ac2eb
--- /dev/null
+++ b/.changeset/cart-get-respects-cartid.md
@@ -0,0 +1,5 @@
+---
+"@shopify/hydrogen": patch
+---
+
+Fix `cartGetDefault` not respecting the `cartId` argument. The returned function accepts an optional `cartInput.cartId`, but the implementation called `getCartId()` unconditionally and then early-returned `null` when that was falsy — so a caller passing `{cartId: '…'}` with no cart cookie got `null` back. The resolved cart id now prefers `cartInput.cartId` before falling back to `getCartId()`, and the GraphQL `variables` spread order was swapped so the resolved cart id always wins over an explicit `{cartId: undefined}` in the input.
diff --git a/packages/hydrogen/src/cart/queries/cartGetDefault.test.ts b/packages/hydrogen/src/cart/queries/cartGetDefault.test.ts
index 1f2f2b4b98..4d9795cea5 100644
--- a/packages/hydrogen/src/cart/queries/cartGetDefault.test.ts
+++ b/packages/hydrogen/src/cart/queries/cartGetDefault.test.ts
@@ -53,6 +53,28 @@ describe('cartGetDefault', () => {
     expect(result).toHaveProperty('id', 'gid://shopify/Cart/c1-456');
   });
 
+  it('should use the cartId passed in when getCartId returns undefined', async () => {
+    const cartGet = cartGetDefault({
+      storefront: mockCreateStorefrontClient(),
+      getCartId: () => undefined,
+    });
+
+    const result = await cartGet({cartId: 'gid://shopify/Cart/c1-456'});
+
+    expect(result).toHaveProperty('id', 'gid://shopify/Cart/c1-456');
+  });
+
+  it('should fall back to getCartId when cartInput.cartId is explicitly undefined', async () => {
+    const cartGet = cartGetDefault({
+      storefront: mockCreateStorefrontClient(),
+      getCartId: () => CART_ID,
+    });
+
+    const result = await cartGet({cartId: undefined});
+
+    expect(result).toHaveProperty('id', CART_ID);
+  });
+
   describe('run with customerAccount option', () => {
     it('should add logged_in search param to checkout link if customer is logged in', async () => {
       const cartGet = cartGetDefault({
diff --git a/packages/hydrogen/src/cart/queries/cartGetDefault.ts b/packages/hydrogen/src/cart/queries/cartGetDefault.ts
index 71ddee722b..12ca9f15c4 100644
--- a/packages/hydrogen/src/cart/queries/cartGetDefault.ts
+++ b/packages/hydrogen/src/cart/queries/cartGetDefault.ts
@@ -63,14 +63,14 @@ export function cartGetDefault({
   cartFragment,
 }: CartGetOptions): CartGetFunction {
   return async (cartInput?: CartGetProps) => {
-    const cartId = getCartId();
+    const cartId = cartInput?.cartId ?? getCartId();
 
     if (!cartId) return null;
 
     const [isCustomerLoggedIn, {cart, errors}] = await Promise.all([
       customerAccount ? customerAccount.isLoggedIn() : false,
       storefront.query<{cart: Cart | null}>(CART_QUERY(cartFragment), {
-        variables: {cartId, ...cartInput},
+        variables: {...cartInput, cartId},
         cache: storefront.CacheNone(),
       }),
     ]);