Joseph Francis Hagan (better known as Philadelphia Jack O'Brien) (January 17, 1878 – November 12, 1942) was an American world light heavyweight boxing champion in 1905 when he defeated Bob Fitzsimmons for the universal world title. Rather than defending his title, O'Brien instead abandoned it in order to fight at heavyweight. Nat Fleischer, founder and editor of The Ring Magazine, ranked O'Brien as the No. 2 All-Time Light Heavyweight, and famed boxing promoter Charley Rose ranked him as the No. 3 All-Time Light Heavyweight.
O'Brien was inducted into the Ring Magazine hall of fame in 1968, the World Boxing Hall of Fame in 1987, and the International Boxing Hall of Fame in 1994.