diff --git a/.changeset/underline-nav-css-overflow.md b/.changeset/underline-nav-css-overflow.md
new file mode 100644
index 00000000000..e504c01bcc1
--- /dev/null
+++ b/.changeset/underline-nav-css-overflow.md
@@ -0,0 +1,5 @@
+---
+"@primer/react": minor
+---
+
+Refactors `UnderlineNav` overflow handling to use CSS-based overflow detection instead of JavaScript width measurements, eliminating layout shift (CLS) issues and improving performance. The overflow menu is now implemented with `ActionMenu`, and item registration uses a descendant registry instead of the `React.Children` API. Consumer-facing changes: items can now be wrapped in fragments or wrapper components; the current item may appear in the overflow menu when the viewport is narrow; and the overflow menu button is right-aligned.