It's supposed to have been Abner Doubleday, although there has been some controversy about that. But until someone proves otherwise, I'll consider Doubleday as the one who invented my favorite game.
Gloria, thanks..my source says Alexander Cartwright. Here is further info:
For years, most people believed that Abner Doubleday invented the game in Cooperstown, New York. In actuality, it evolved from English games such as rounders and cricket and came to be called "town ball" in the United States. Young Alexander Cartwright had played that game, and in 1845, he and his team, the Knickerbocker Base Ball Club, wrote down specific new rules, such as three strikes for an out and three outs for an inning. Cartwright was inducted to the National Baseball Hall of Fame in 1938. Doubleday has never been inducted.