diff --git a/.changeset/humble-jars-prove.md b/.changeset/humble-jars-prove.md
new file mode 100644
index 0000000000..28f7484efd
--- /dev/null
+++ b/.changeset/humble-jars-prove.md
@@ -0,0 +1,5 @@
+---
+"@radix-ui/react-navigation-menu": major
+---
+
+Fix missing data-state attribute on NavigationMenuContent when used with forceMount and NavigationMenuViewport
diff --git a/apps/storybook/stories/navigation-menu.stories.tsx b/apps/storybook/stories/navigation-menu.stories.tsx
index 8493dfb42f..36eb440d7f 100644
--- a/apps/storybook/stories/navigation-menu.stories.tsx
+++ b/apps/storybook/stories/navigation-menu.stories.tsx
@@ -4,6 +4,42 @@ import styles from './navigation-menu.stories.module.css';
 
 export default { title: 'Components/NavigationMenu' };
 
+export const ForceMountWithViewport = () => {
+  return (
+    <StoryFrame>
+      <NavigationMenu.Root>
+        <NavigationMenu.List className={styles.mainList}>
+          <NavigationMenu.Item>
+            <TriggerWithIndicator>Products</TriggerWithIndicator>
+            {/* forceMount keeps content mounted; Viewport routes it via ViewportContentMounter */}
+            <NavigationMenu.Content
+              forceMount
+              className={styles.viewportContent}
+              style={{ width: 400 }}
+            >
+              <LinkGroup items={['Item One', 'Item Two', 'Item Three']} />
+            </NavigationMenu.Content>
+          </NavigationMenu.Item>
+
+          <NavigationMenu.Item>
+            <TriggerWithIndicator>Company</TriggerWithIndicator>
+            <NavigationMenu.Content
+              forceMount
+              className={styles.viewportContent}
+              style={{ width: 300 }}
+            >
+              <LinkGroup items={['About', 'Careers']} />
+            </NavigationMenu.Content>
+          </NavigationMenu.Item>
+        </NavigationMenu.List>
+
+        {/* Viewport is the key — this triggers the ViewportContentMounter path */}
+        <NavigationMenu.Viewport />
+      </NavigationMenu.Root>
+    </StoryFrame>
+  );
+};
+
 export const Basic = () => {
   return (
     <StoryFrame>
diff --git a/packages/react/navigation-menu/src/navigation-menu.tsx b/packages/react/navigation-menu/src/navigation-menu.tsx
index e363f5e9b8..f839bdd85e 100644
--- a/packages/react/navigation-menu/src/navigation-menu.tsx
+++ b/packages/react/navigation-menu/src/navigation-menu.tsx
@@ -764,6 +764,7 @@ const NavigationMenuContent = React.forwardRef<
   const itemContext = useNavigationMenuItemContext(CONTENT_NAME, props.__scopeNavigationMenu);
   const composedRefs = useComposedRefs(itemContext.contentRef, forwardedRef);
   const open = itemContext.value === context.value;
+  const openState = getOpenState(open);
 
   const commonProps = {
     value: itemContext.value,
@@ -794,7 +795,12 @@ const NavigationMenuContent = React.forwardRef<
       />
     </Presence>
   ) : (
-    <ViewportContentMounter forceMount={forceMount} {...commonProps} ref={composedRefs} />
+    <ViewportContentMounter
+      data-state={openState}
+      forceMount={forceMount}
+      {...commonProps}
+      ref={composedRefs}
+    />
   );
 });