John Dwight Chesbro (June 5, 1874 – November 6, 1931) was a Major League Baseball (MLB) pitcher. Nicknamed "Happy Jack", Chesbro played for the Pittsburgh Pirates (1899–1902), the New York Highlanders (1903–1909), and the Boston Red Sox (1909). Chesbro finished his career with a win-loss record of 198-132, an earned run average of 2.68, and 1,265 strikeouts. His 41 wins during the 1904 season remains an American League record. Though some pitchers have won more games in some seasons prior to 1901, historians demarcating 1901 as the beginning of 'modern-era' major league baseball refer to and credit Jack Chesbro and his 1904 win-total as the modern era major league record and its holder. Some view Chesbro's 41 wins in a season as an unbreakable record.

Chesbro's 1904 pitching totals of 51 games started and 48 complete games also fall into the same historical category as his 1904 wins total, as they are all-time American League single-season records. These 1904 single-season totals for games started and complete games, as the wins total, are also the most recorded by a pitcher in either the American or National League, since the beginning of the twentieth century and the co-existence of the American and National Leagues as major leagues. If one demarcates 1901 as the beginning of major league baseball's modern era, Jack Chesbro holds the modern era major league historical single-season records for wins by a pitcher (41), games started by a pitcher (51), and complete games pitched (48).

Chesbro was elected to the National Baseball Hall of Fame in 1946 by the Veterans Committee, though he had received little consideration from the Baseball Writers' Association of America (BBWAA). Some baseball historians consider the 1946 election a mistake, and believe that Chesbro was elected solely on the basis of his 1904 season.

He was born John D. Chesbro on June 5, 1874 in Houghtonville, a village in North Adams, Massachusetts. He was the fourth of five children of Chad Brown Chesebrough, a shoemaker, and Martha Jane Fralensburgh.

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