John Dwight Chesbro (June 5, 1874 – November 6, 1931) was an American professional baseball pitcher. Nicknamed "Happy Jack", Chesbro played for the Pittsburgh Pirates (1899–1902), the New York Highlanders (1903–1909), and the Boston Red Sox (1909) of Major League Baseball (MLB). Chesbro finished his career with a 198–132 win–loss record, a 2.68 earned run average, 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.