James Jackson Jeffries (April 15, 1875 – March 3, 1953) was an American professional boxer and World Heavyweight Champion. He was known for his enormous strength and stamina.

Using a technique taught to him by his trainer, former Welterweight and Middleweight Champion Tommy Ryan, Jeffries fought out of a crouch with his left arm extended forward. He was able to absorb tremendous punishment while wearing his opponents down. A natural left-hander, he possessed one-punch knockout power in his left hook, and brawled his way to the top of the rankings.