Donald Thomas Regan (December 21, 1918 – June 10, 2003) was the 66th United States Secretary of the Treasury from 1981 to 1985 and the White House Chief of Staff from 1985 to 1987 under Ronald Reagan. In the Reagan administration, he advocated "Reaganomics" and tax cuts as a means to create jobs and to stimulate production.

Earlier in his life, he had studied at Harvard University before he served in the United States Marine Corps, achieving the rank of lieutenant colonel. In 1946 he started to work for Merrill Lynch. He served as its chairman and CEO from 1971 to 1980.