Robert B. Willumstad is a former Chairman and CEO of the American International Group (AIG).

He was born in Brooklyn and grew up on Long Island. His alma mater is Adelphi University. He and his wife Carol reside in New York City, and have two adult daughters and two grandchildren.

Under Sanford I. Weill, Willumstad helped to steer the 1998 merger of Travelers Group and Citicorp. From 2000 to 2003, he was the Chairman and CEO of Citigroup's Global Consumer Group. He was named President of Citigroup in 2002, and assumed the post of Chief Operating Officer in 2003, while continuing to serve as CEO and President of Citibank North America. Some suggested that he had been passed over for CEO in favour of Charles Prince, despite being in charge of Citi's largest business.