diff --git a/src/sidebar/components/Annotation/AnnotationEditor.tsx b/src/sidebar/components/Annotation/AnnotationEditor.tsx
index 1eccfc1095c..6c6fa38c9dc 100644
--- a/src/sidebar/components/Annotation/AnnotationEditor.tsx
+++ b/src/sidebar/components/Annotation/AnnotationEditor.tsx
@@ -28,6 +28,7 @@ import { useSidebarStore } from '../../store';
 import type { Draft } from '../../store/modules/drafts';
 import MarkdownEditor from '../MarkdownEditor';
 import TagEditor from '../TagEditor';
+import { useUnsavedChanges } from '../hooks/unsaved-changes';
 import AnnotationLicense from './AnnotationLicense';
 import AnnotationPublishControl from './AnnotationPublishControl';
 
@@ -75,6 +76,12 @@ function AnnotationEditor({
   const text = draft.text;
   const isEmpty = !text && !tags.length;
 
+  // Warn user if they try to close the tab while there is an open, non-empty
+  // draft.
+  //
+  // WARNING: This does not work in all browsers. See hook docs for details.
+  useUnsavedChanges(!isEmpty);
+
   const onEditTags = useCallback(
     (tags: string[]) => {
       store.createDraft(draft.annotation, { ...draft, tags });
diff --git a/src/sidebar/components/Annotation/test/AnnotationEditor-test.js b/src/sidebar/components/Annotation/test/AnnotationEditor-test.js
index 6f54cd8dd88..9eb2155ab1a 100644
--- a/src/sidebar/components/Annotation/test/AnnotationEditor-test.js
+++ b/src/sidebar/components/Annotation/test/AnnotationEditor-test.js
@@ -18,6 +18,7 @@ describe('AnnotationEditor', () => {
   let fakeSettings;
   let fakeToastMessenger;
   let fakeGroupsService;
+  let fakeUseUnsavedChanges;
 
   let fakeStore;
 
@@ -74,8 +75,13 @@ describe('AnnotationEditor', () => {
       defaultAuthority: sinon.stub().returns(''),
     };
 
+    fakeUseUnsavedChanges = sinon.stub();
+
     $imports.$mock(mockImportedComponents());
     $imports.$mock({
+      '../hooks/unsaved-changes': {
+        useUnsavedChanges: fakeUseUnsavedChanges,
+      },
       '../../store': { useSidebarStore: () => fakeStore },
       '../../helpers/theme': { applyTheme: fakeApplyTheme },
     });
@@ -279,6 +285,19 @@ describe('AnnotationEditor', () => {
       assert.notCalled(fakeAnnotationsService.save);
     });
 
+    it('warns if user closes tab while there is a non-empty draft', () => {
+      // If the draft is empty, the warning is disabled.
+      const wrapper = createComponent();
+      assert.calledWith(fakeUseUnsavedChanges, false);
+
+      // If the draft changes to non-empty, the warning is enabled.
+      fakeUseUnsavedChanges.resetHistory();
+      const draft = fixtures.defaultDraft();
+      draft.text = 'something is here';
+      wrapper.setProps({ draft });
+      assert.calledWith(fakeUseUnsavedChanges, true);
+    });
+
     describe('handling publish options', () => {
       it('sets the publish control to disabled if the annotation is empty', () => {
         // default draft has no tags or text
diff --git a/src/sidebar/components/hooks/test/unsaved-changes-test.js b/src/sidebar/components/hooks/test/unsaved-changes-test.js
new file mode 100644
index 00000000000..93112520cef
--- /dev/null
+++ b/src/sidebar/components/hooks/test/unsaved-changes-test.js
@@ -0,0 +1,62 @@
+import { mount } from '@hypothesis/frontend-testing';
+
+import { useUnsavedChanges, hasUnsavedChanges } from '../unsaved-changes';
+
+function TestUseUnsavedChanges({ unsaved, fakeWindow }) {
+  useUnsavedChanges(unsaved, fakeWindow);
+  return <div />;
+}
+
+describe('useUnsavedChanges', () => {
+  let fakeWindow;
+
+  function dispatchBeforeUnload() {
+    const event = new Event('beforeunload', { cancelable: true });
+    fakeWindow.dispatchEvent(event);
+    return event;
+  }
+
+  function createWidget(unsaved) {
+    return mount(
+      <TestUseUnsavedChanges fakeWindow={fakeWindow} unsaved={unsaved} />,
+    );
+  }
+
+  beforeEach(() => {
+    // Use a dummy window to avoid triggering any handlers that respond to
+    // "beforeunload" on the real window.
+    fakeWindow = new EventTarget();
+  });
+
+  it('does not increment unsaved-changes count if argument is false', () => {
+    createWidget(false);
+    assert.isFalse(hasUnsavedChanges());
+  });
+
+  it('does not register "beforeunload" handler if argument is false', () => {
+    createWidget(false);
+    const event = dispatchBeforeUnload();
+    assert.isFalse(event.defaultPrevented);
+  });
+
+  it('increments unsaved-changes count if argument is true', () => {
+    const wrapper = createWidget(true);
+    assert.isTrue(hasUnsavedChanges());
+    wrapper.unmount();
+    assert.isFalse(hasUnsavedChanges());
+  });
+
+  it('registers "beforeunload" handler if argument is true', () => {
+    const wrapper = createWidget(true);
+    const event = dispatchBeforeUnload();
+    assert.isTrue(event.defaultPrevented);
+
+    // We don't test `event.returnValue` here because it returns `false` after
+    // assignment in Chrome, even though the handler assigns it `true`.
+
+    // Unmount the widget, this should remove the handler.
+    wrapper.unmount();
+    const event2 = dispatchBeforeUnload();
+    assert.isFalse(event2.defaultPrevented);
+  });
+});
diff --git a/src/sidebar/components/hooks/unsaved-changes.ts b/src/sidebar/components/hooks/unsaved-changes.ts
new file mode 100644
index 00000000000..a86162e9173
--- /dev/null
+++ b/src/sidebar/components/hooks/unsaved-changes.ts
@@ -0,0 +1,55 @@
+import { useEffect } from 'preact/hooks';
+
+/** Count of components with unsaved changes. */
+let unsavedCount = 0;
+
+function preventUnload(e: BeforeUnloadEvent) {
+  // `preventDefault` is the modern API for preventing unload.
+  e.preventDefault();
+
+  // Setting `returnValue` to a truthy value is a legacy method needed for
+  // Firefox. Note that in Chrome, reading `returnValue` will return false
+  // afterwards.
+  e.returnValue = true;
+}
+
+/**
+ * Return true if any active components have indicated they have unsaved changes
+ * using {@link useUnsavedChanges}.
+ */
+export function hasUnsavedChanges() {
+  return unsavedCount > 0;
+}
+
+/**
+ * Hook that registers the current component as having unsaved changes that
+ * would be lost in the event of a navigation.
+ *
+ * WARNING: As of May 2025, this works in Chrome and Firefox, but not Safari.
+ * See https://github.com/hypothesis/support/issues/59#issuecomment-2886335334.
+ *
+ * @param hasUnsavedChanges - True if current component has unsaved changes
+ * @param window_ - Test seam
+ */
+export function useUnsavedChanges(
+  hasUnsavedChanges: boolean,
+  /* istanbul ignore next - test seam */
+  window_ = window,
+) {
+  useEffect(() => {
+    if (!hasUnsavedChanges) {
+      return () => {};
+    }
+
+    unsavedCount += 1;
+    if (unsavedCount === 1) {
+      window_.addEventListener('beforeunload', preventUnload);
+    }
+    return () => {
+      unsavedCount -= 1;
+      if (unsavedCount === 0) {
+        window_.removeEventListener('beforeunload', preventUnload);
+      }
+    };
+  }, [hasUnsavedChanges, window_]);
+}