James "Big Jim" Colosimo (born Giacomo Colosimo) (February 16, 1878 – May 11, 1920), also known as "Diamond Jim," was an Italian-American Mafia crime boss who immigrated from Calabria, Italy in 1895, and built a criminal empire in Chicago based on prostitution, gambling, and racketeering. He gained power through petty crime and by heading a chain of brothels. From about 1902 until his murder in 1920, he led a gang that became known after his death as the Chicago Outfit.