diff --git a/.changeset/underline-wrapper-force-no-border.md b/.changeset/underline-wrapper-force-no-border.md
new file mode 100644
index 00000000000..2d218d95f74
--- /dev/null
+++ b/.changeset/underline-wrapper-force-no-border.md
@@ -0,0 +1,5 @@
+---
+'@primer/react': patch
+---
+
+UnderlineNav: Prevent external borders from incorrectly triggering the overflow menu
diff --git a/packages/react/src/internal/components/UnderlineTabbedInterface.module.css b/packages/react/src/internal/components/UnderlineTabbedInterface.module.css
index 7beddcdff52..04f1df2ccc0 100644
--- a/packages/react/src/internal/components/UnderlineTabbedInterface.module.css
+++ b/packages/react/src/internal/components/UnderlineTabbedInterface.module.css
@@ -25,7 +25,7 @@
 
     /* Pin the vertical box model so external box-sizing/border/padding overrides can't change the height budget and incorrectly trigger the overflow "more" menu */
     box-sizing: border-box !important;
-    border-block: 0 !important;
+    border: 0 !important;
     padding-block: var(--base-size-8) 0 !important;
 
     .UnderlineItemList {