William Brennan may refer to:
- William J. Brennan, Jr. (1906–1997), former Associate Justice of the United States Supreme Court
- William Brennan (baseball) (born 1963), Major League Baseball pitcher
- William Brennan (Australian politician) (1865–1937), member of the New South Wales Parliament
- Bill Brennan (boxer) (1893–1924), American boxer
- Bill Brennan (umpire) (1880–1933), Major League Baseball umpire and college football coach
- Billy Brennan (born 1934), ice hockey player