John William McCormack (December 21, 1891 - November 22, 1980) was an American politician from Boston, Massachusetts.

He was a member of United States House of Representatives from 1928 until he retired from political life in 1971. McCormack served as House Majority Leader thrice, the first time from 1940 to 1947, the second time from 1949 to 1953, and again from 1955 to 1962. He served as Speaker of the House of Representatives from 1961 until 1971. He was a member of the Democratic Party.

Between the assassination of John F. Kennedy on November 22, 1963 and the swearing-in of Hubert Humphrey as Vice-President on January 20, 1965, McCormack was the first person in the line of succession for the Presidency, holding the same position the new president, Lyndon Johnson had in relation to Kennedy when vice-president, and he received Secret Service protection. When LBJ died in 1973, McCormack recalled his experiences serving as next-in-line in an article he wrote for The Boston Globe.

In 1983 the University of Massachusetts Boston established the John W. McCormack Institute of Public Affairs , named in McCormack's honor. In 2003 it was expanded into the John W. McCormack Graduate School of Policy Studies .