diff --git a/.changeset/underlinenav-rerender-on-children-change.md b/.changeset/underlinenav-rerender-on-children-change.md
new file mode 100644
index 00000000000..9260a5ccd26
--- /dev/null
+++ b/.changeset/underlinenav-rerender-on-children-change.md
@@ -0,0 +1,5 @@
+---
+'@primer/react': patch
+---
+
+Fix `UnderlineNav` not re-rendering when child tabs are added or removed.
diff --git a/packages/react/src/UnderlineNav/UnderlineNav.features.stories.tsx b/packages/react/src/UnderlineNav/UnderlineNav.features.stories.tsx
index 6b35a2c739b..39c1749ba87 100644
--- a/packages/react/src/UnderlineNav/UnderlineNav.features.stories.tsx
+++ b/packages/react/src/UnderlineNav/UnderlineNav.features.stories.tsx
@@ -13,6 +13,7 @@ import {
 } from '@primer/octicons-react'
 import type {Meta} from '@storybook/react-vite'
 import {UnderlineNav} from './index'
+import {Button} from '../Button'
 import {INITIAL_VIEWPORTS} from 'storybook/viewport'
 
 const meta = {
@@ -154,3 +155,29 @@ export const VariantFlush = () => {
     </UnderlineNav>
   )
 }
+
+export const DynamicChildren = () => {
+  const [showItem, setShowItem] = React.useState(false)
+
+  return (
+    <div>
+      <Button
+        onClick={() => {
+          setShowItem(prev => !prev)
+        }}
+      >
+        {showItem ? 'Hide item' : 'Show item'}
+      </Button>
+      <UnderlineNav aria-label="Repository">
+        <UnderlineNav.Item href="#" aria-current="page">
+          Code
+        </UnderlineNav.Item>
+        <UnderlineNav.Item href="#">Pull requests</UnderlineNav.Item>
+        <UnderlineNav.Item href="#">Actions</UnderlineNav.Item>
+        <UnderlineNav.Item href="#">Projects</UnderlineNav.Item>
+        <UnderlineNav.Item href="#">Wiki</UnderlineNav.Item>
+        {showItem && <UnderlineNav.Item href="#">Another</UnderlineNav.Item>}
+      </UnderlineNav>
+    </div>
+  )
+}
diff --git a/packages/react/src/UnderlineNav/UnderlineNav.test.tsx b/packages/react/src/UnderlineNav/UnderlineNav.test.tsx
index 1ab761f8e34..ca256c8cc86 100644
--- a/packages/react/src/UnderlineNav/UnderlineNav.test.tsx
+++ b/packages/react/src/UnderlineNav/UnderlineNav.test.tsx
@@ -1,6 +1,6 @@
 import {describe, expect, it, vi} from 'vitest'
-import type React from 'react'
-import {render, screen} from '@testing-library/react'
+import React from 'react'
+import {render, screen, waitFor} from '@testing-library/react'
 import userEvent from '@testing-library/user-event'
 import {
   CodeIcon,
@@ -240,6 +240,54 @@ describe('UnderlineNav', () => {
     const textSpan = item.querySelector('[data-component="text"]')
     expect(textSpan).toHaveAttribute('data-content', 'Simple Text')
   })
+
+  // Regression test for https://github.com/github/primer/issues/6389
+  it('re-renders when a conditionally rendered child tab is toggled', async () => {
+    const Example = () => {
+      const [showItem, setShowItem] = React.useState(false)
+      return (
+        <div>
+          <button type="button" onClick={() => setShowItem(prev => !prev)}>
+            {showItem ? 'hide item' : 'show item'}
+          </button>
+          <UnderlineNav aria-label="Repository">
+            <UnderlineNav.Item href="#" aria-current="page">
+              Code
+            </UnderlineNav.Item>
+            <UnderlineNav.Item href="#">Pull requests</UnderlineNav.Item>
+            <UnderlineNav.Item href="#">Actions</UnderlineNav.Item>
+            <UnderlineNav.Item href="#">Projects</UnderlineNav.Item>
+            <UnderlineNav.Item href="#">Wiki</UnderlineNav.Item>
+            {showItem && <UnderlineNav.Item href="#">Another</UnderlineNav.Item>}
+          </UnderlineNav>
+        </div>
+      )
+    }
+
+    render(<Example />)
+    const toggle = screen.getByRole('button', {name: 'show item'})
+    const user = userEvent.setup()
+
+    // Initially the conditional tab is not rendered
+    expect(screen.queryByText('Another')).not.toBeInTheDocument()
+
+    // Toggle on: the new tab should appear in the DOM (either in the visible list
+    // or the overflow menu) without the nav being remounted. `findByText` waits
+    // for the ResizeObserver-driven re-render that runs after `childrenSignature`
+    // changes (the callback is async in real browsers like chromium).
+    await user.click(toggle)
+    expect(await screen.findByText('Another')).toBeInTheDocument()
+    // Existing tabs are still present
+    expect(screen.getByText('Code')).toBeInTheDocument()
+    expect(screen.getByText('Pull requests')).toBeInTheDocument()
+
+    // Toggle off: the tab should be removed
+    await user.click(screen.getByRole('button', {name: 'hide item'}))
+    await waitFor(() => {
+      expect(screen.queryByText('Another')).not.toBeInTheDocument()
+    })
+    expect(screen.getByText('Code')).toBeInTheDocument()
+  })
 })
 
 describe('Keyboard Navigation', () => {
diff --git a/packages/react/src/UnderlineNav/UnderlineNav.tsx b/packages/react/src/UnderlineNav/UnderlineNav.tsx
index 21774554c2f..00bd84341a4 100644
--- a/packages/react/src/UnderlineNav/UnderlineNav.tsx
+++ b/packages/react/src/UnderlineNav/UnderlineNav.tsx
@@ -175,6 +175,14 @@ export const UnderlineNav = forwardRef(
       menuItems: [],
     })
 
+    // Track the set of children so the overflow effect re-runs whenever children are
+    // added or removed (not just when the nav element resizes). Keys are sorted so that
+    // simply reordering existing children doesn't trigger an unnecessary recalculation.
+    const childrenSignature = validChildren
+      .map(child => String(child.key ?? ''))
+      .sort()
+      .join('|')
+
     // Make sure to have the fresh props data for list items when children are changed (keeping aria-current up-to-date)
     const listItems = responsiveProps.items.map(item => {
       return validChildren.find(child => child.key === item.key) ?? item
@@ -294,19 +302,23 @@ export const UnderlineNav = forwardRef(
 
     useOnOutsideClick({onClickOutside: closeOverlay, containerRef, ignoreClickRefs: [moreMenuBtnRef]})
 
-    useResizeObserver((resizeObserverEntries: ResizeObserverEntry[]) => {
-      const navWidth = resizeObserverEntries[0].contentRect.width
-      const moreMenuWidth = moreMenuRef.current?.getBoundingClientRect().width ?? 0
-      navWidth !== 0 &&
-        overflowEffect(
-          navWidth,
-          moreMenuWidth,
-          validChildren,
-          childWidthArray,
-          noIconChildWidthArray,
-          updateListAndMenu,
-        )
-    }, navRef as RefObject<HTMLElement>)
+    useResizeObserver(
+      (resizeObserverEntries: ResizeObserverEntry[]) => {
+        const navWidth = resizeObserverEntries[0].contentRect.width
+        const moreMenuWidth = moreMenuRef.current?.getBoundingClientRect().width ?? 0
+        navWidth !== 0 &&
+          overflowEffect(
+            navWidth,
+            moreMenuWidth,
+            validChildren,
+            childWidthArray,
+            noIconChildWidthArray,
+            updateListAndMenu,
+          )
+      },
+      navRef as RefObject<HTMLElement>,
+      [childrenSignature],
+    )
 
     // Compute menuInlineStyles if needed
     let menuInlineStyles: React.CSSProperties = {...baseMenuInlineStyles}