John Brennan may refer to:
Public officials
Jack Brennan (1937–2023), U.S. Marine officer and aide of Richard Nixon
John O. Brennan (born 1955), former CIA director
John P. Brennan (1864–1943), Democratic politician in the U.S. state of Ohio
John Brennan Hussey (born 1934), former mayor of Shreveport, Louisiana
John Joseph Brennan (1913–1976), Northern Ireland politician
John Brennan (Irish senator) (1901–1977), Fianna Fáil politician
John A. Brennan Jr. (born 1945), American lobbyist and Massachusetts politician
Sports
J. J. Brennan (1887–1943), Irish hurler
John Brennan (Derry Gaelic footballer) (born 1942), Gaelic football manager and player
John Brennan (Kerry Gaelic footballer), former Gaelic footballer
Jack Brennan (footballer, born 1892) (1892–1942), English footballer for Bradford City and Manchester City
John Brennan (American football) (1913–1982), American football player
Jack Brennan (baseball) (1862–1914), American baseball catcher
John Brennan (athlete) (1879–1964), American track and field athlete
Artists
Johnny Brennan (born 1961), American actor, screenwriter and voice actor
John Wolf Brennan (born 1954), Irish pianist, organist and composer
Other people
John J. Brennan (businessman) (born 1954), chairman and CEO of The Vanguard Group
John Calder Brennan (1908–1996), U.S. historian
John Brennan (doctor) (c. 1768–1830), Irish doctor
Sidney Czira (1889–1974), Irish journalist and revolutionary known by her pen name "John Brennan"
John N. H. Brennan (1914–2010), Irish author, huntsman and solicitor
John P. Brennan (priest) (c. 1836–1889), first American Catholic priest to declare bankruptcy
John W. Brennan (born c. 1968), United States Army general