diff --git a/.changeset/pos-cart-update-line-item-quantity.md b/.changeset/pos-cart-update-line-item-quantity.md
new file mode 100644
index 0000000000..57f7d20903
--- /dev/null
+++ b/.changeset/pos-cart-update-line-item-quantity.md
@@ -0,0 +1,6 @@
+---
+'@shopify/ui-extensions': minor
+'@shopify/ui-extensions-tester': patch
+---
+
+Add `cart.updateLineItemQuantity` to the POS Cart API for API version 2026-10. The method updates one line item's quantity while preserving its properties, discounts, and selling plans.
diff --git a/packages/ui-extensions-tester/src/point-of-sale/README.md b/packages/ui-extensions-tester/src/point-of-sale/README.md
index a30a20cc41..81dc8ef8c6 100644
--- a/packages/ui-extensions-tester/src/point-of-sale/README.md
+++ b/packages/ui-extensions-tester/src/point-of-sale/README.md
@@ -49,9 +49,27 @@ extension.shopify.storage = createStorage({
 });
 ```
 
-## 🔒 Mocking mutation return values
+## 🔒 Mocking cart mutations
 
-Replace mutation functions with `vi.fn()` and use `createResult()` to build typed return values. The first argument is the mutation name; the second is an optional result override.
+The target API mock includes asynchronous stubs for Cart API mutations, including `updateLineItemQuantity`. Replace a stub with a spy when you need to verify a call:
+
+```ts
+const updateLineItemQuantity = vi.spyOn(
+  extension.shopify.cart,
+  'updateLineItemQuantity',
+);
+
+await extension.shopify.cart.updateLineItemQuantity(
+  'line-item-uuid',
+  2,
+);
+
+expect(
+  updateLineItemQuantity,
+).toHaveBeenCalledWith('line-item-uuid', 2);
+```
+
+For mutations that return data, replace the function with `vi.fn()` and use `createResult()` to build typed return values. The first argument is the mutation name; the second is an optional result override.
 
 ```ts
 import {
diff --git a/packages/ui-extensions-tester/src/point-of-sale/factories.ts b/packages/ui-extensions-tester/src/point-of-sale/factories.ts
index 65a7596eeb..b581e0663e 100644
--- a/packages/ui-extensions-tester/src/point-of-sale/factories.ts
+++ b/packages/ui-extensions-tester/src/point-of-sale/factories.ts
@@ -224,6 +224,7 @@ function createMockCartApi(): CartApi {
       removeCustomer: async () => {},
       addCustomSale: async () => '',
       addLineItem: async () => '',
+      updateLineItemQuantity: async () => {},
       removeLineItem: async () => {},
       addCartProperties: async () => {},
       removeCartProperties: async () => {},
diff --git a/packages/ui-extensions-tester/src/tests/pos-resolution-targets.test.ts b/packages/ui-extensions-tester/src/tests/pos-resolution-targets.test.ts
index b3de09332e..599c42a9b0 100644
--- a/packages/ui-extensions-tester/src/tests/pos-resolution-targets.test.ts
+++ b/packages/ui-extensions-tester/src/tests/pos-resolution-targets.test.ts
@@ -17,6 +17,7 @@ describe('pos.cart.validations.resolution.render', () => {
 
     expect(api.cart.current).toBeDefined();
     expect(typeof api.cart.addLineItem).toBe('function');
+    expect(typeof api.cart.updateLineItemQuantity).toBe('function');
     expect(api.scanner).toBeDefined();
   });
 
diff --git a/packages/ui-extensions/docs/surfaces/point-of-sale/reference/examples/cart-api/update-line-item-quantity.jsx b/packages/ui-extensions/docs/surfaces/point-of-sale/reference/examples/cart-api/update-line-item-quantity.jsx
new file mode 100644
index 0000000000..072528f62b
--- /dev/null
+++ b/packages/ui-extensions/docs/surfaces/point-of-sale/reference/examples/cart-api/update-line-item-quantity.jsx
@@ -0,0 +1,23 @@
+import {render} from 'preact';
+
+export default async () => {
+  render(<Extension />, document.body);
+};
+
+const Extension = () => {
+  return (
+    <s-tile
+      heading="My App"
+      subheading="Call cart function"
+      onClick={() => {
+        const [lineItem] = shopify.cart.current.value.lineItems;
+        if (lineItem) {
+          shopify.cart.updateLineItemQuantity(
+            lineItem.uuid,
+            lineItem.quantity + 1,
+          );
+        }
+      }}
+    />
+  );
+};
diff --git a/packages/ui-extensions/src/surfaces/point-of-sale/api/cart-api/cart-api.ts b/packages/ui-extensions/src/surfaces/point-of-sale/api/cart-api/cart-api.ts
index d1aba4d46d..fbaf469a6b 100644
--- a/packages/ui-extensions/src/surfaces/point-of-sale/api/cart-api/cart-api.ts
+++ b/packages/ui-extensions/src/surfaces/point-of-sale/api/cart-api/cart-api.ts
@@ -138,6 +138,20 @@ export interface MutableCartApiContent {
     options?: AddLineItemOptions,
   ): Promise<string>;
 
+  /**
+   * Set the quantity of an existing line item identified by its `UUID`, preserving the line item's properties, discounts, and selling plans. This is equivalent to a merchant adjusting the quantity on the native cart line.
+   *
+   * If POS has split the line into multiple allocations that share the same `UUID` (for example, lines split across delivery methods), the target line is ambiguous and the call throws instead of guessing.
+   *
+   * Only available on API version `2026-10` and later.
+   *
+   * @param uuid the UUID of the line item to update
+   * @param quantity the new absolute quantity; must be an integer of 1 or greater. To remove a line item, use `removeLineItem` instead.
+   * @returns A promise that resolves after the cart state reflects the change.
+   * @throws {Error} if the line item is not found, the `UUID` matches multiple split-line allocations, the quantity is invalid, the cart is not editable because it is a return or exchange, or the POS app version does not support this method.
+   */
+  updateLineItemQuantity(uuid: string, quantity: number): Promise<void>;
+
   /**
    * Remove a specific line item from the cart using its `UUID`. The line item will be completely removed from the cart along with any associated discounts, properties, or selling plans.
    *