James Jackson Jeffries (born April 15, 1875 in Carroll, Ohio, United States – died March 3, 1953 in Burbank, California) was a world heavyweight boxing champion.

Jeffries was the prototype for the modern athlete. He stood 6' 21/2 tall and weighed about 225 lbs in his prime. Despite his bulk, Jeffries, who was not a trained sprinter, could run the 100 yard dash in a little over 10 seconds. The world record in 1900 for the 100 meter dash was 10.8 seconds. While working as a sparring partner for James J. Corbett, Jeffries consistently outsprinted Corbett. Corbett, who was skilled enough to play professional baseball, was an outstanding all around athlete and had never lost a footrace to any of his campmates. Jeffies was also nimble and could high jump over 6 feet.

His greatest assets, however, were his enormous strength and stamina. Jeffries fought out of a crouch with his left arm extended forward. He was able to absorb tremendous punishment while wearing his opponents down. In addition, Jeffries was a terrific puncher who could floor an opponent with a straight left hand. He possessed one punch knockout power in his left hook.

In 1891, his family moved from their Ohio farm to Los Angeles, California where the powerfully built and athletic teenager boxed as an amateur until age twenty when he started fighting professionally. As a pro, Jeffries never lost a fight and on June 9, 1899 in Brooklyn, New York he defeated Bob Fitzsimmons to win the Heavyweight championship of the world. That August, he embarked on a tour of Europe putting on exhibition fights for the fans. A master promoter, as early as 1899 he made several motion pictures recreating portions of his championship fight plus parts from some of his other bouts and films of some of his exhibition matches.

Jeffries took on any and all comers but was such a strong champion, few contenders wanted to go up against him. During his reign as champion, Jeffries defended his title seven times, including two victories over former champion James J. Corbett. He retired undefeated in May of 1905, giving up his championship. He served as a referee for the next few years, including the bout in which Marvin Hart defeated Jack Root to win the World Heavyweight Championship he had vacated.

Five years after retiring, Jeffries made a comeback and on July 4, 1910 at Reno, Nevada, he fought champion Jack Johnson for the heavyweight championship. Johnson, who had won the title in 1908 to become the first African-American champion, defeated Jeffries in 15 rounds. Unfortuantely, Jeffries is best remembered for this fight.

Johnson's reign as heavyweight champion provoked racial hostility and spawned the "great white hope" era in boxing. Johnson, who was a skilled and fearless champion, easily defeated the "white hopes" who challenged him. The newspapers of the day began clamoring for the undefeated Jeffries to come out of retirement to reclaim the heavyweight championship for the white race. Although, Jeffries hadn't fought in 6 years, was 36 years old and had ballooned to over three hundred pounds, he allowed himself to be persuaded to accept the match.

It didn't take long for Jeffries to realize he had made a big mistake. Johnson easily controlled him and appeared able to end the bout whenever he pleased. It was obvious to all that this was not the same Jeffries who had reigned as champion. He was rusty, and had lost over 100 lbs. in preparing for the fight. This ordeal weakened him and he was unable to bull Johnson around, as he had every other ring opponent he faced. Jeffries, who had been known for his seemingly superhuman stamina, faded fast under the mid day Reno heat. In short, it was no contest.

Jeffries, however, made no excuses. After the fight he stated that he never would have been able to defeat Johnson, even if they had fought in his prime. Interestingly, Johnson, after his ring career was over, consistently named Jeffries as the greatest heavyweight of all time.

In his later years, Jeffries trained boxers and worked as a fight promoter. He promoted many fights out of a structure known as Jeffries Barn. Jeffries Barn is now part of Knott's Berry Farm, a Southern California amusement park. On his passing in 1953, he was interred in the Inglewood Park Cemetery in Inglewood, California.

An all-around fighter with a devastating punch, many consider him one of the great heavyweight champions of all time. James J. Jeffries was elected to the International Boxing Hall of Fame in 1990.