William Brennan may refer to:
William Brennan (Australian politician) (1865–1937), member of the New South Wales Parliament William Brennan (baseball) (born 1963), American baseball pitcher William C. Brennan (1918–2000), New York politician and judge William J. Brennan Jr. (1906–1997), justice of the United States Supreme Court William John Brennan (1938–2013), Australian Roman Catholic bishop William Joseph Brennan (bishop) (1904–1975), Roman Catholic priest in Australia Will Brennan (born 1998), American baseball player Willis Brennan (1893–1950), American football player Willy Brennan (died 1804), Irish highwayman Bill Brennan (umpire) (1880–1933), Major League Baseball umpire and college football coach Bill Brennan (boxer) (1893–1924), American boxer Bill Brennan (activist) (born 1968), candidate for governor of New Jersey in 2017 Bill Brennan (journalist), American sports journalist Bill Brennan (Nebraska politician) (1922–1979), member of the Nebraska Legislature Billy Brennan (1934–2020), British ice hockey player