Alexander Joys Cartwright Jr. (April 17, 1820 July 12, 1892) was a founding member of the New York Knickerbockers Base Ball Club in the 1840s. Although he was an inductee of the Baseball Hall of Fame and he was sometimes referred to as a "father of baseball," the importance of his role in the development of the game has been disputed.

The rules of the modern game were long considered to have been based on the Knickerbocker Rules developed in 1845 by Cartwright and a committee from the Knickerbockers. However, later research called this scenario into question. The myth of Abner Doubleday having invented baseball was believed by many, but in Cooperstown in 1939, the myth was debunked. Cartwright was inducted into the Hall of Fame as a pioneering contributor to the game, 46 years after his death. Although it has been stated that Cartwright was officially declared the inventor of the modern game of baseball by the 83rd United States Congress on June 3, 1953, the Congressional Record, the House Journal, and the Senate Journal from June 3, 1953, did not mention Cartwright.

The history of baseball in the United States dates to the 18th century, when boys and amateur enthusiasts played a baseball-like game by their own informal rules using homemade equipment. The popularity of the sport grew and amateur men's ball clubs were formed in the 1830–50s. Semi-professional baseball clubs followed in the 1860s, and the first professional leagues arrived in the post-American Civil War 1870s.