Rosenthal, who helped oversee the integration of Morgan Stanley with Smith Barney, will continue to serve as a senior adviser at the Wall Street firm, the memo said.

A Morgan Stanley spokesman confirmed the contents of the memo.

Rosenthal served with other top Morgan Stanley executives on its operating committee and helped to lead many of the firm's efforts in technology, operations and cyber security.

He joined Morgan Stanley in 2008 from real estate firm Tishman Speyer, where he had served as chief financial officer. He became chief operating officer in 2011.

Morgan Stanley has not yet named a replacement for Rosenthal.

(Reporting by Olivia Oran in New York; Editing by Bernard Orr and Cynthia Osterman)

By Olivia Oran