William Gibbs McAdoo (October 31, 1863–February 1, 1941) was a U.S. Senator,United States Secretary of the Treasury and director of the United States Railroad Administration.

McAdoo was the son-in-law of President Woodrow Wilson, marrying his daughter, Eleanor Randolph Wilson , at the White House on May 7, 1914.

He served as Secretary of the Treasury under Wilson from 1913–1918, and was instrumental in forging the legislation that founded the Federal Reserve. After leaving the Wilson Cabinet, he ran twice for the Democratic nomination for President, losing to James M. Cox in 1920, and to John W. Davis in 1924, even though in both years he led on the first ballot. He served as Senator for California from 1933–1938. He and Eleanor were divorced in 1934.

McAdoo was a "Dry" with respect to Prohibition, and was the favored candidate of the Ku Klux Klan in 1924 when the other front-runner appeared to be the Catholic Al Smith of New York. McAdoo took a payment of $25,000 from oil executive Edward Doheny in connection with the Teapot Dome scandal, but returned it once he discovered Doheny's links with Secretary of the Interior Albert Bacon Fall.

McAdoo once famously said that the speeches of President Warren Harding "leave the impression of an army of pompous phrases moving over the landscape in search of an idea; sometimes these meandering words would actually capture a straggling thought and bear it triumphantly, a prisoner in their midst, until it died of servitude and overwork."