diff --git a/web/modals/threads/members/change-member-role-modal.css b/web/modals/threads/members/change-member-role-modal.css
--- a/web/modals/threads/members/change-member-role-modal.css
+++ b/web/modals/threads/members/change-member-role-modal.css
@@ -48,3 +48,9 @@
 .roleModalBackButton {
   padding: 14px 32px;
 }
+
+.roleModalErrorMessage {
+  color: var(--error);
+  display: flex;
+  justify-content: center;
+}
diff --git a/web/modals/threads/members/change-member-role-modal.react.js b/web/modals/threads/members/change-member-role-modal.react.js
--- a/web/modals/threads/members/change-member-role-modal.react.js
+++ b/web/modals/threads/members/change-member-role-modal.react.js
@@ -8,6 +8,8 @@
   changeThreadMemberRolesActionTypes,
 } from 'lib/actions/thread-actions.js';
 import { useModalContext } from 'lib/components/modal-provider.react.js';
+import { otherUsersButNoOtherAdmins } from 'lib/selectors/thread-selectors.js';
+import { roleIsAdminRole } from 'lib/shared/thread-utils.js';
 import type { RelativeMemberInfo, ThreadInfo } from 'lib/types/thread-types';
 import {
   useDispatchActionPromise,
@@ -19,6 +21,7 @@
 import Button, { buttonThemes } from '../../../components/button.react.js';
 import Dropdown from '../../../components/dropdown.react.js';
 import UserAvatar from '../../../components/user-avatar.react.js';
+import { useSelector } from '../../../redux/redux-utils.js';
 import Modal from '../../modal.react.js';
 import UnsavedChangesModal from '../../unsaved-changes-modal.react.js';
 
@@ -32,6 +35,10 @@
   const { pushModal, popModal } = useModalContext();
   const dispatchActionPromise = useDispatchActionPromise();
   const callChangeThreadMemberRoles = useServerCall(changeThreadMemberRoles);
+  const [showErrorMessage, setShowErrorMessage] = React.useState(false);
+  const otherUsersButNoOtherAdminsValue = useSelector(
+    otherUsersButNoOtherAdmins(threadInfo.id),
+  );
   const modalName = 'Change Role';
 
   const roleOptions = React.useMemo(
@@ -62,12 +69,39 @@
     pushModal(<UnsavedChangesModal />);
   }, [initialSelectedRole, popModal, pushModal, selectedRole]);
 
+  const errorMessage = React.useMemo(() => {
+    if (!showErrorMessage) {
+      return null;
+    }
+
+    return (
+      <>
+        <div className={css.roleModalErrorMessage}>Cannot change role.</div>
+        <div className={css.roleModalErrorMessage}>
+          There must be at least one admin at any given time.
+        </div>
+      </>
+    );
+  }, [showErrorMessage]);
+
   const onSave = React.useCallback(() => {
     if (selectedRole === initialSelectedRole) {
       popModal();
       return;
     }
 
+    const memberIsAdmin = memberInfo.role
+      ? roleIsAdminRole(threadInfo.roles[memberInfo.role])
+      : true;
+
+    // Handle case where the sole admin's role is being changed.
+    if (otherUsersButNoOtherAdminsValue && memberIsAdmin) {
+      setShowErrorMessage(true);
+      return;
+    }
+
+    setShowErrorMessage(false);
+
     const createChangeThreadMemberRolesPromise = () => {
       invariant(selectedRole, 'Expected selected role to be set');
       return callChangeThreadMemberRoles(
@@ -84,12 +118,13 @@
     popModal();
   }, [
     callChangeThreadMemberRoles,
+    otherUsersButNoOtherAdminsValue,
     dispatchActionPromise,
     initialSelectedRole,
-    memberInfo.id,
+    memberInfo,
     popModal,
     selectedRole,
-    threadInfo.id,
+    threadInfo,
   ]);
 
   return (
@@ -111,6 +146,7 @@
           setActiveSelection={setSelectedRole}
         />
       </div>
+      {errorMessage}
       <div className={css.roleModalActionButtons}>
         <Button
           variant="outline"