William Brennan
William Brennan may refer to:
- William Brennan (Australian politician), member of the New South Wales Parliament
- William Brennan (baseball), American baseball pitcher
- William C. Brennan, New York politician and judge
- William J. Brennan Jr., justice of the United States Supreme Court
- William John Brennan, Australian Roman Catholic bishop
- William Joseph Brennan (bishop), Roman Catholic priest in Australia
- Will Brennan, American baseball player
- Willis Brennan, American football player
- Willy Brennan, Irish highwayman
- Bill Brennan (umpire), Major League Baseball umpire and college football coach
- Bill Brennan (boxer), American boxer
- Bill Brennan (activist), candidate for Governor of New Jersey in 2017
- Bill Brennan (journalist), American sports journalist
- Billy Brennan, British ice hockey player