British-born boxer, the first fighter to hold the world boxing championship in three weight divisions.

A New Zealand resident as a young man, Fitzsimmons went to the United States in 1890, having already established a reputation as a fighter. He won the world middleweight title (under the Queensberry rules) by knocking out Nonpareil Jack Dempsey (Dempsey, Nonpareil Jack) in 13 rounds in New Orleans, Louisiana, on January 14, 1891. He resigned this championship on March 17, 1897, when he won the heavyweight title by knocking out Gentleman Jim Corbett (Corbett, James J.) in 14 rounds at Carson City, Nevada. Fitzsimmons knocked Corbett out with the first display of his solar plexus punch.

Fitzsimmons lost the heavyweight championship to James Jackson Jeffries (Jeffries, James Jackson) on an 11th-round knockout at Coney Island in New York City on June 9, 1899. He came back to win the light heavyweight championship on November 25, 1903, in San Francisco, when he defeated George Gardner in 20 rounds. He lost this title to Philadelphia Jack O'Brien on a 13th-round knockout in San Francisco on December 20, 1905. He continued fighting until 1914. Fitzsimmons weighed only about 170 pounds but had the chest and shoulder development of a much larger man, which allowed him to beat boxers who had a large weight advantage over him. Fitzsimmons was elected into The Ring magazine's Boxing Hall of Fame in 1954.