diff --git a/.changeset/bitter-cougars-deny.md b/.changeset/bitter-cougars-deny.md
new file mode 100644
index 00000000000..92d445b2541
--- /dev/null
+++ b/.changeset/bitter-cougars-deny.md
@@ -0,0 +1,5 @@
+---
+'@primer/react': patch
+---
+
+AvatarStack: Border was incorrectly applying to elements that were not `Avatar` children.
\ No newline at end of file
diff --git a/.changeset/evil-zebras-train.md b/.changeset/evil-zebras-train.md
new file mode 100644
index 00000000000..826567afec7
--- /dev/null
+++ b/.changeset/evil-zebras-train.md
@@ -0,0 +1,5 @@
+---
+'@primer/react': patch
+---
+
+AvatarStack: The square prop was not applying to individual `Avatar` components.
\ No newline at end of file
diff --git a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-dark-high-contrast-linux.png b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-dark-high-contrast-linux.png
index 90eb690a62d..86c185f36ac 100644
Binary files a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-dark-high-contrast-linux.png and b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-dark-high-contrast-linux.png differ
diff --git a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-colorblind-linux.png b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-colorblind-linux.png
index 259c1323df3..a6e828ef2f8 100644
Binary files a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-colorblind-linux.png and b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-colorblind-linux.png differ
diff --git a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-high-contrast-linux.png b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-high-contrast-linux.png
index 0812851cc87..73ce393f601 100644
Binary files a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-high-contrast-linux.png and b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-high-contrast-linux.png differ
diff --git a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-linux.png b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-linux.png
index 259c1323df3..a6e828ef2f8 100644
Binary files a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-linux.png and b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-linux.png differ
diff --git a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-tritanopia-linux.png b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-tritanopia-linux.png
index 259c1323df3..a6e828ef2f8 100644
Binary files a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-tritanopia-linux.png and b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Link-Wrappers-light-tritanopia-linux.png differ
diff --git a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-dark-colorblind-linux.png b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-dark-colorblind-linux.png
new file mode 100644
index 00000000000..e58d2543331
Binary files /dev/null and b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-dark-colorblind-linux.png differ
diff --git a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-dark-dimmed-linux.png b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-dark-dimmed-linux.png
new file mode 100644
index 00000000000..db54a67910b
Binary files /dev/null and b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-dark-dimmed-linux.png differ
diff --git a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-dark-high-contrast-linux.png b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-dark-high-contrast-linux.png
new file mode 100644
index 00000000000..35981cccadc
Binary files /dev/null and b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-dark-high-contrast-linux.png differ
diff --git a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-dark-linux.png b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-dark-linux.png
new file mode 100644
index 00000000000..e58d2543331
Binary files /dev/null and b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-dark-linux.png differ
diff --git a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-dark-tritanopia-linux.png b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-dark-tritanopia-linux.png
new file mode 100644
index 00000000000..e58d2543331
Binary files /dev/null and b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-dark-tritanopia-linux.png differ
diff --git a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-light-colorblind-linux.png b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-light-colorblind-linux.png
new file mode 100644
index 00000000000..79044ca85e2
Binary files /dev/null and b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-light-colorblind-linux.png differ
diff --git a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-light-high-contrast-linux.png b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-light-high-contrast-linux.png
new file mode 100644
index 00000000000..79044ca85e2
Binary files /dev/null and b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-light-high-contrast-linux.png differ
diff --git a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-light-linux.png b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-light-linux.png
new file mode 100644
index 00000000000..79044ca85e2
Binary files /dev/null and b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-light-linux.png differ
diff --git a/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-light-tritanopia-linux.png b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-light-tritanopia-linux.png
new file mode 100644
index 00000000000..79044ca85e2
Binary files /dev/null and b/.playwright/snapshots/components/AvatarStack.test.ts-snapshots/AvatarStack-With-Square-Avatars-light-tritanopia-linux.png differ
diff --git a/e2e/components/AvatarStack.test.ts b/e2e/components/AvatarStack.test.ts
index c66e4bf22d7..273f4946c98 100644
--- a/e2e/components/AvatarStack.test.ts
+++ b/e2e/components/AvatarStack.test.ts
@@ -43,6 +43,10 @@ const stories: Array<{title: string; id: string}> = [
     title: 'With Link Wrappers',
     id: 'components-avatarstack-dev--with-link-wrappers',
   },
+  {
+    title: 'With Square Avatars',
+    id: 'components-avatarstack-features--with-square-avatars',
+  },
 ]
 
 test.describe('AvatarStack', () => {
diff --git a/packages/react/src/AvatarStack/AvatarStack.features.stories.tsx b/packages/react/src/AvatarStack/AvatarStack.features.stories.tsx
index 9141f2665b3..968161e1e68 100644
--- a/packages/react/src/AvatarStack/AvatarStack.features.stories.tsx
+++ b/packages/react/src/AvatarStack/AvatarStack.features.stories.tsx
@@ -62,6 +62,15 @@ export const CustomSizeOnChildren = () => (
   </AvatarStack>
 )
 
+export const WithSquareAvatars = () => (
+  <AvatarStack shape="square">
+    <Avatar alt="Primer logo" src="https://avatars.githubusercontent.com/primer" />
+    <Avatar alt="GitHub logo" src="https://avatars.githubusercontent.com/github" />
+    <Avatar alt="Atom logo" src="https://avatars.githubusercontent.com/atom" />
+    <Avatar alt="GitHub Desktop logo" src="https://avatars.githubusercontent.com/desktop" />
+  </AvatarStack>
+)
+
 // the smallest size of the children avatars will be used at each breakpoint
 export const CustomSizeOnChildrenResponsive = () => (
   <AvatarStack>
diff --git a/packages/react/src/AvatarStack/AvatarStack.module.css b/packages/react/src/AvatarStack/AvatarStack.module.css
index a5e1bc3a9a9..b455ec447c8 100644
--- a/packages/react/src/AvatarStack/AvatarStack.module.css
+++ b/packages/react/src/AvatarStack/AvatarStack.module.css
@@ -36,7 +36,7 @@
   }
 
   &:where([data-avatar-count='1'][data-shape='circle']) {
-    .AvatarItem {
+    .AvatarItem:where([data-component='Avatar']) {
       /* stylelint-disable-next-line primer/box-shadow */
       box-shadow: 0 0 0 var(--avatar-border-width) var(--avatar-borderColor);
     }
@@ -143,6 +143,10 @@
     box-shadow: 0 0 0 var(--avatar-border-width) transparent;
   }
 
+  &:not([data-component='Avatar']):not(:has([data-square])) {
+    border-radius: 50%;
+  }
+
   .AvatarStack:where([data-shape='square']) &:is(img) {
     /* stylelint-disable-next-line primer/box-shadow */
     box-shadow: 1px 0 rgb(255, 255, 255, 1);
diff --git a/packages/react/src/AvatarStack/AvatarStack.tsx b/packages/react/src/AvatarStack/AvatarStack.tsx
index 9661161a24c..ae400a90876 100644
--- a/packages/react/src/AvatarStack/AvatarStack.tsx
+++ b/packages/react/src/AvatarStack/AvatarStack.tsx
@@ -13,7 +13,7 @@ const transformChildren = (children: React.ReactNode, shape: AvatarStackProps['s
     if (!React.isValidElement(child)) return child
     return React.cloneElement(child, {
       ...child.props,
-      square: shape === 'square' ? true : undefined,
+      square: (shape === 'square' ? true : undefined) || child.props.square,
       className: clsx(child.props.className, 'pc-AvatarItem', classes.AvatarItem),
     })
   })