diff --git a/web/chat/thread-menu.react.js b/web/chat/thread-menu.react.js
--- a/web/chat/thread-menu.react.js
+++ b/web/chat/thread-menu.react.js
@@ -26,6 +26,7 @@
 import MenuItem from '../components/menu-item.react';
 import Menu from '../components/menu.react';
 import SidebarListModal from '../modals/chat/sidebar-list-modal.react';
+import SidebarPromoteModal from '../modals/chat/sidebar-promote-modal.react';
 import { useModalContext } from '../modals/modal-provider.react';
 import ConfirmLeaveThreadModal from '../modals/threads/confirm-leave-thread-modal.react';
 import ThreadMembersModal from '../modals/threads/members/members-modal.react';
@@ -96,6 +97,18 @@
     [setModal, threadInfo],
   );
 
+  const onClickPromoteSidebarToThread = React.useCallback(
+    () =>
+      setModal(
+        <SidebarPromoteModal
+          threadInfo={threadInfo}
+          onClose={clearModal}
+          onConfirm={onPromoteSidebar}
+        />,
+      ),
+    [setModal, threadInfo, clearModal, onPromoteSidebar],
+  );
+
   const sidebarItem = React.useMemo(() => {
     if (!hasSidebars) {
       return null;
@@ -202,10 +215,10 @@
         key="promote"
         text="Promote Thread"
         icon="message-square-lines"
-        onClick={onPromoteSidebar}
+        onClick={onClickPromoteSidebarToThread}
       />
     );
-  }, [onPromoteSidebar]);
+  }, [onClickPromoteSidebarToThread]);
 
   const menuItems = React.useMemo(() => {
     const notificationsItem = (
diff --git a/web/modals/chat/sidebar-promote-modal.react.js b/web/modals/chat/sidebar-promote-modal.react.js
new file mode 100644
--- /dev/null
+++ b/web/modals/chat/sidebar-promote-modal.react.js
@@ -0,0 +1,42 @@
+// @flow
+
+import * as React from 'react';
+
+import type { ThreadInfo } from 'lib/types/thread-types';
+
+import Button from '../../components/button.react';
+import Modal from '../modal.react';
+
+type Props = {
+  +onClose: () => void,
+  +onConfirm: () => void,
+  +threadInfo: ThreadInfo,
+};
+
+function SidebarPromoteModal(props: Props): React.Node {
+  const { threadInfo, onClose, onConfirm } = props;
+  const { uiName } = threadInfo;
+
+  const handleConfirm = React.useCallback(() => {
+    onConfirm();
+    onClose();
+  }, [onClose, onConfirm]);
+
+  return (
+    <Modal name="Promote Thread" icon="warning-circle" onClose={onClose}>
+      <div>
+        <p>{`Are you sure you want to promote "${uiName}"?
+        "This promoting a sidebar to a full thread can't be undone."`}</p>
+
+        <Button onClick={handleConfirm} type="submit" variant="danger">
+          Promote to Full Thread
+        </Button>
+        <Button onClick={onConfirm} type="submit" variant="secondary">
+          Cancel
+        </Button>
+      </div>
+    </Modal>
+  );
+}
+
+export default SidebarPromoteModal;