diff --git a/pages/flashbar/stack-collapse-control.page.tsx b/pages/flashbar/stack-collapse-control.page.tsx
new file mode 100644
index 0000000000..088a204323
--- /dev/null
+++ b/pages/flashbar/stack-collapse-control.page.tsx
@@ -0,0 +1,72 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+import * as React from 'react';
+
+import Box from '~components/box';
+import Flashbar, { FlashbarProps } from '~components/flashbar';
+import SpaceBetween from '~components/space-between';
+
+import ScreenshotArea from '../utils/screenshot-area';
+
+const noop = () => void 0;
+
+const i18nStrings: FlashbarProps.I18nStrings = {
+  ariaLabel: 'Notifications',
+  notificationBarText: 'Notifications',
+  notificationBarAriaLabel: 'View all notifications',
+  errorIconAriaLabel: 'Error',
+  warningIconAriaLabel: 'Warning',
+  successIconAriaLabel: 'Success',
+  infoIconAriaLabel: 'Information',
+  inProgressIconAriaLabel: 'In progress',
+};
+
+const items: FlashbarProps.MessageDefinition[] = [
+  {
+    id: 'success',
+    type: 'success',
+    statusIconAriaLabel: 'Success',
+    header: 'Instance created',
+    dismissible: true,
+    onDismiss: noop,
+    dismissLabel: 'Dismiss',
+  },
+  {
+    id: 'warning',
+    type: 'warning',
+    statusIconAriaLabel: 'Warning',
+    header: 'Something weird may have happened...',
+  },
+  {
+    id: 'error',
+    type: 'error',
+    statusIconAriaLabel: 'Error',
+    header: 'Unrecoverable error',
+    content: 'It all broke, like, really bad.',
+  },
+];
+
+export default function FlashbarStackCollapseControl() {
+  const [expanded, setExpanded] = React.useState<boolean | null>(null);
+
+  return (
+    <>
+      <h1>Flashbar stacked group: defaultExpanded + onToggle</h1>
+      <SpaceBetween size="m">
+        <Box>
+          Last toggle event:{' '}
+          <span id="toggle-state">{expanded === null ? '(none yet)' : expanded ? 'expanded' : 'collapsed'}</span>
+        </Box>
+        <ScreenshotArea disableAnimations={true}>
+          <Flashbar
+            stackItems={true}
+            defaultExpanded={true}
+            onToggle={event => setExpanded(event.detail.expanded)}
+            i18nStrings={i18nStrings}
+            items={items}
+          />
+        </ScreenshotArea>
+      </SpaceBetween>
+    </>
+  );
+}
diff --git a/src/__tests__/snapshot-tests/__snapshots__/documenter.test.ts.snap b/src/__tests__/snapshot-tests/__snapshots__/documenter.test.ts.snap
index b54c1b56ab..1014da4d5c 100644
--- a/src/__tests__/snapshot-tests/__snapshots__/documenter.test.ts.snap
+++ b/src/__tests__/snapshot-tests/__snapshots__/documenter.test.ts.snap
@@ -14297,7 +14297,27 @@ If you want to clear the selection, use empty array.",
 exports[`Components definition for flashbar matches the snapshot: flashbar 1`] = `
 {
   "dashCaseName": "flashbar",
-  "events": [],
+  "events": [
+    {
+      "cancelable": false,
+      "description": "Called when the user expands or collapses the stacked notifications group
+by activating the notification bar. Only fires when \`stackItems\` is set to
+\`true\`. The event \`detail\` contains the new value of the expanded state.",
+      "detailInlineType": {
+        "name": "FlashbarProps.ToggleDetail",
+        "properties": [
+          {
+            "name": "expanded",
+            "optional": false,
+            "type": "boolean",
+          },
+        ],
+        "type": "object",
+      },
+      "detailType": "FlashbarProps.ToggleDetail",
+      "name": "onToggle",
+    },
+  ],
   "functions": [],
   "name": "Flashbar",
   "properties": [
@@ -14308,6 +14328,15 @@ exports[`Components definition for flashbar matches the snapshot: flashbar 1`] =
       "optional": true,
       "type": "string",
     },
+    {
+      "description": "Determines whether the stacked notifications group is initially expanded.
+Only has an effect when \`stackItems\` is set to \`true\`. Defaults to \`false\`
+(the group starts collapsed). This property is applied on the initial
+render only; afterwards the expanded state is managed by the component.",
+      "name": "defaultExpanded",
+      "optional": true,
+      "type": "boolean",
+    },
     {
       "description": "An object containing all the necessary localized strings required by the component. The object should contain:
 
diff --git a/src/flashbar/__tests__/collapsible.test.tsx b/src/flashbar/__tests__/collapsible.test.tsx
index 9dac776908..e447599c6d 100644
--- a/src/flashbar/__tests__/collapsible.test.tsx
+++ b/src/flashbar/__tests__/collapsible.test.tsx
@@ -644,6 +644,48 @@ describe('Collapsible Flashbar', () => {
 });
 
 // Entire interactive element including the counter and the actual <button/> element
+describe('Collapsible Flashbar expand/collapse control', () => {
+  it('starts collapsed by default (defaultExpanded not set)', () => {
+    const flashbar = renderFlashbar();
+    expect(flashbar.findItems()).toHaveLength(1);
+    expect(flashbar.findToggleButton()!.getElement()).toHaveAttribute('aria-expanded', 'false');
+  });
+
+  it('starts expanded when defaultExpanded is true', () => {
+    const flashbar = renderFlashbar({ defaultExpanded: true });
+    expect(flashbar.findItems()).toHaveLength(defaultItems.length);
+    expect(flashbar.findToggleButton()!.getElement()).toHaveAttribute('aria-expanded', 'true');
+  });
+
+  it('starts collapsed when defaultExpanded is false', () => {
+    const flashbar = renderFlashbar({ defaultExpanded: false });
+    expect(flashbar.findItems()).toHaveLength(1);
+    expect(flashbar.findToggleButton()!.getElement()).toHaveAttribute('aria-expanded', 'false');
+  });
+
+  it('fires onToggle with expanded=true when the user expands the stack', () => {
+    const onToggle = jest.fn();
+    const flashbar = renderFlashbar({ onToggle });
+    findNotificationBar(flashbar)!.click();
+    expect(onToggle).toHaveBeenCalledTimes(1);
+    expect(onToggle).toHaveBeenCalledWith(expect.objectContaining({ detail: { expanded: true } }));
+  });
+
+  it('fires onToggle with expanded=false when the user collapses the stack', () => {
+    const onToggle = jest.fn();
+    const flashbar = renderFlashbar({ defaultExpanded: true, onToggle });
+    findNotificationBar(flashbar)!.click();
+    expect(onToggle).toHaveBeenCalledTimes(1);
+    expect(onToggle).toHaveBeenCalledWith(expect.objectContaining({ detail: { expanded: false } }));
+  });
+
+  it('does not fire onToggle on initial render', () => {
+    const onToggle = jest.fn();
+    renderFlashbar({ defaultExpanded: true, onToggle });
+    expect(onToggle).not.toHaveBeenCalled();
+  });
+});
+
 function findNotificationBar(flashbar: FlashbarWrapper): HTMLElement | undefined {
   const element = Array.from(flashbar.getElement().children).find(
     element => element instanceof HTMLElement && element.tagName !== 'UL'
diff --git a/src/flashbar/collapsible-flashbar.tsx b/src/flashbar/collapsible-flashbar.tsx
index 8e045c3fb4..131317cffc 100644
--- a/src/flashbar/collapsible-flashbar.tsx
+++ b/src/flashbar/collapsible-flashbar.tsx
@@ -15,6 +15,7 @@ import InternalIcon from '../icon/internal';
 import { animate, getDOMRects } from '../internal/animate';
 import { Transition } from '../internal/components/transition';
 import { getVisualContextClassname } from '../internal/components/visual-context';
+import { fireNonCancelableEvent } from '../internal/events';
 import customCssProps from '../internal/generated/custom-css-properties';
 import { useDebounceCallback } from '../internal/hooks/use-debounce-callback';
 import { useEffectOnUpdate } from '../internal/hooks/use-effect-on-update';
@@ -50,11 +51,17 @@ const maxNonCollapsibleItems = 1;
 
 const resizeListenerThrottleDelay = 100;
 
-export default function CollapsibleFlashbar({ items, style, ...restProps }: InternalFlashbarProps) {
+export default function CollapsibleFlashbar({
+  items,
+  style,
+  defaultExpanded,
+  onToggle,
+  ...restProps
+}: InternalFlashbarProps) {
   const visibleItems = useFlashbarVisibility(items);
   const [enteringItems, setEnteringItems] = useState<ReadonlyArray<FlashbarProps.MessageDefinition>>([]);
   const [exitingItems, setExitingItems] = useState<ReadonlyArray<FlashbarProps.MessageDefinition>>([]);
-  const [isFlashbarStackExpanded, setIsFlashbarStackExpanded] = useState(false);
+  const [isFlashbarStackExpanded, setIsFlashbarStackExpanded] = useState(defaultExpanded ?? false);
 
   const getElementsToAnimate = useCallback(() => {
     const flashElements = isFlashbarStackExpanded ? expandedItemRefs.current : collapsedItemRefs.current;
@@ -104,7 +111,9 @@ export default function CollapsibleFlashbar({ items, style, ...restProps }: Inte
     if (!isReducedMotion) {
       prepareAnimations();
     }
-    setIsFlashbarStackExpanded(prev => !prev);
+    const nextExpanded = !isFlashbarStackExpanded;
+    setIsFlashbarStackExpanded(nextExpanded);
+    fireNonCancelableEvent(onToggle, { expanded: nextExpanded });
   }
 
   const debouncedFocus = useDebounceCallback(focusFlashById, FOCUS_DEBOUNCE_DELAY);
diff --git a/src/flashbar/interfaces.ts b/src/flashbar/interfaces.ts
index 8d455fd800..f0447eb579 100644
--- a/src/flashbar/interfaces.ts
+++ b/src/flashbar/interfaces.ts
@@ -5,6 +5,7 @@ import React from 'react';
 import { ButtonProps } from '../button/interfaces';
 import { ErrorContext } from '../types/analytics';
 import { BaseComponentProps } from '../types/base-component';
+import { NonCancelableEventHandler } from '../types/events';
 
 export namespace FlashbarProps {
   export interface MessageDefinition {
@@ -34,6 +35,10 @@ export namespace FlashbarProps {
     crossServicePersistence?: boolean;
   }
 
+  export interface ToggleDetail {
+    expanded: boolean;
+  }
+
   export interface I18nStrings {
     ariaLabel?: string;
     errorIconAriaLabel?: string;
@@ -197,6 +202,21 @@ export interface FlashbarProps extends BaseComponentProps {
    */
   stackItems?: boolean;
 
+  /**
+   * Determines whether the stacked notifications group is initially expanded.
+   * Only has an effect when `stackItems` is set to `true`. Defaults to `false`
+   * (the group starts collapsed). This property is applied on the initial
+   * render only; afterwards the expanded state is managed by the component.
+   */
+  defaultExpanded?: boolean;
+
+  /**
+   * Called when the user expands or collapses the stacked notifications group
+   * by activating the notification bar. Only fires when `stackItems` is set to
+   * `true`. The event `detail` contains the new value of the expanded state.
+   */
+  onToggle?: NonCancelableEventHandler<FlashbarProps.ToggleDetail>;
+
   /**
    * An object containing all the necessary localized strings required by the component. The object should contain:
    *