diff --git a/.changeset/underline-wrapper-force-box-sizing.md b/.changeset/underline-wrapper-force-box-sizing.md
new file mode 100644
index 00000000000..fa03ed4b145
--- /dev/null
+++ b/.changeset/underline-wrapper-force-box-sizing.md
@@ -0,0 +1,5 @@
+---
+'@primer/react': patch
+---
+
+Pin `box-sizing` with `!important` on the underline tabbed interface wrapper in wrap mode so external overrides can no longer offset its height budget and incorrectly trigger the overflow "more" menu
diff --git a/packages/react/src/internal/components/UnderlineTabbedInterface.module.css b/packages/react/src/internal/components/UnderlineTabbedInterface.module.css
index 8e55f1a796d..7beddcdff52 100644
--- a/packages/react/src/internal/components/UnderlineTabbedInterface.module.css
+++ b/packages/react/src/internal/components/UnderlineTabbedInterface.module.css
@@ -23,8 +23,8 @@
     overflow: hidden;
     max-height: var(--control-xlarge-size);
 
-    /* Pin the vertical box model so external border/padding overrides can't change the height budget and incorrectly trigger the overflow "more" menu */
-    box-sizing: border-box;
+    /* 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;
     padding-block: var(--base-size-8) 0 !important;