diff --git a/.changeset/underline-item-list-flex-grow.md b/.changeset/underline-item-list-flex-grow.md
new file mode 100644
index 00000000000..cae26e4cd3e
--- /dev/null
+++ b/.changeset/underline-item-list-flex-grow.md
@@ -0,0 +1,5 @@
+---
+'@primer/react': patch
+---
+
+Fix `UnderlineNav` tab list not filling the full width of the underline nav in Safari
diff --git a/packages/react/src/internal/components/UnderlineTabbedInterface.module.css b/packages/react/src/internal/components/UnderlineTabbedInterface.module.css
index 238b18146df..18972c96f7b 100644
--- a/packages/react/src/internal/components/UnderlineTabbedInterface.module.css
+++ b/packages/react/src/internal/components/UnderlineTabbedInterface.module.css
@@ -24,6 +24,7 @@
     max-height: var(--control-xlarge-size);
 
     .UnderlineItemList {
+      flex-grow: 1;
       flex-wrap: wrap;
       overflow: hidden;
     }