Donald Thomas Regan[a] (December 21, 1918 – June 10, 2003) was an American government official and business executive who 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.

Regan studied at Harvard University before he served in the U.S. Marine Corps, achieving the rank of lieutenant colonel.

In the Reagan administration, Regan advocated "Reaganomics" and tax cuts as a means to create jobs and to stimulate production.

Regan was one of the original directors of the Securities Investor Protection Corporation and was vice chairman of the New York Stock Exchange from 1973 to 1975.

President Ronald Reagan selected Donald Regan in 1981 to serve as treasury secretary, marking him as a spokesman for his economic policies, dubbed "Reaganomics".

[1] The Tower Commission, established by President Reagan to investigate the scandal, concluded that Regan was responsible for the "chaos" that took hold of the White House.

Regan wrote: Virtually every major move and decision the Reagans made during my time as White House Chief of Staff was cleared in advance with a woman in San Francisco [Quigley] who drew up horoscopes to make certain that the planets were in a favorable alignment for the enterprise.

[citation needed] In the portrait of Regan that hangs on the third floor of the treasury, the title of a book in the background reads And the Horse You Rode In On.