Michael Joseph "King" Kelly (December 31, 1857 – November 8, 1894), also commonly known as "$10,000 Kelly," was an American outfielder, catcher, and manager in various professional American baseball leagues including the National League, International Association, Players' League, and the American Association. He spent the majority of his 16-season playing career with the and the Boston Beaneaters. Kelly was a player-manager three times in his career – in 1887 for the Beaneaters, in 1890 leading the Boston Reds to the pennant in the only season of the Players' League's existence, and in 1891 for the Cincinnati Kelly's Killers – before his retirement in 1893. He is also often credited with helping to popularize various strategies as a player such as the hit and run, the hook slide, and the catcher's practice of backing up first base.

In only the second vote since its creation in 1939 the Old Timers Committee (now the Veterans' Committee) elected Kelly to the Baseball Hall of Fame in 1945.

In concluding where to truly give Kelly credit as an innovator, a 2004 book devoted to 19th-century rule bending in baseball—and which came close to exhaustively accounting for all contemporary reporting on various subjects—placed stress on the following: "Kelly's hook slide does sound special, and players probably tried to copy it. Also, he seems to have been the first big leaguer to successfully cut a base (when the usually lone umpire wasn't looking), at least according to the newspaper record." And, "Kelly could have been the first to foul off lots of pitches on purpose. Doing so was a top trick of some Baltimore players of the 1890s. At the turn of the century, that trick was defused when all foul balls began counting as strikes."

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