diff --git a/.changeset/tidy-buttons-smile.md b/.changeset/tidy-buttons-smile.md
new file mode 100644
index 0000000000..789bc4b857
--- /dev/null
+++ b/.changeset/tidy-buttons-smile.md
@@ -0,0 +1,5 @@
+---
+"@cloudflare/kumo": patch
+---
+
+Preserve ButtonGroup's outer corners while an overlay control is open.
diff --git a/packages/kumo/src/components/button-group/button-group.test.tsx b/packages/kumo/src/components/button-group/button-group.test.tsx
index 3ed7449103..a76de92420 100644
--- a/packages/kumo/src/components/button-group/button-group.test.tsx
+++ b/packages/kumo/src/components/button-group/button-group.test.tsx
@@ -62,6 +62,19 @@ describe("ButtonGroup", () => {
     expect(screen.getByRole("group").className).toContain("flex-row");
   });
 
+  it("ignores non-control siblings mounted by an open overlay", () => {
+    render(
+      <ButtonGroup>
+        <Button>One</Button>
+        <Button>Two</Button>
+        <span data-portal-placeholder="" />
+      </ButtonGroup>,
+    );
+    expect(screen.getByRole("group").className).toContain(
+      "[&>*:is(button,a):has(~_:is(button,a))]:rounded-e-none",
+    );
+  });
+
   it("uses logical child-position selectors to join controls", () => {
     render(
       <ButtonGroup>
diff --git a/packages/kumo/src/components/button-group/button-group.tsx b/packages/kumo/src/components/button-group/button-group.tsx
index c9ae60ea3a..f217841172 100644
--- a/packages/kumo/src/components/button-group/button-group.tsx
+++ b/packages/kumo/src/components/button-group/button-group.tsx
@@ -27,7 +27,7 @@ export const KUMO_BUTTON_GROUP_STYLING = {
     // and LinkButton groups retain the correct outer corners. The one-level
     // descendant selectors support Button's tooltip wrapper.
     "[&>*:not(:first-child):is(button,a)]:rounded-s-none",
-    "[&>*:not(:last-child):is(button,a)]:rounded-e-none",
+    "[&>*:is(button,a):has(~_:is(button,a))]:rounded-e-none",
     "[&>*:not(:first-child)>:is(button,a)]:rounded-s-none",
     "[&>*:not(:last-child)>:is(button,a)]:rounded-e-none",
     // Overlap borders/rings by 1px so adjacent buttons share a single seam