John Donato Torrio (born Donato Torrio, Italian: [doˈnaːto ˈtɔrrjo]; January 20, 1882 – April 16, 1957) was an Italian-born mobster who helped build the Chicago Outfit in the 1920s later inherited by his protégé Al Capone. Torrio proposed a National Crime Syndicate in the 1930s and later became an adviser to Lucky Luciano and his Luciano crime family.

Torrio had several nicknames, primarily "The Fox" for his cunning and finesse. The US Treasury official Elmer Irey considered him "the biggest gangster in America" and wrote, "He was the smartest and, I dare say, the best of all the hoodlums. 'Best' referring to talent, not morals." Virgil W. Peterson of the Chicago Crime Commission stated that his "talents as an organizational genius were widely respected by the major gang bosses in the New York City area". Crime journalist Herbert Asbury affirmed: "As an organizer and administrator of underworld affairs, Johnny Torrio is unsurpassed in the annals of American crime; he was probably the nearest thing to a real mastermind that this country has yet produced".

Torrio was born in Irsina (then known as Montepeloso), Basilicata, in Southern Italy, to Tommaso Torrio and Maria Carluccio originally from Altamura, Apulia. When he was two his father, a railway employee, died in a work accident; shortly after, Torrio immigrated to James Street on the Lower East Side of New York City with his widowed mother in December 1884. She later remarried.

His first jobs were as a porter and bouncer in Manhattan. While he was a teenager, he joined a street gang together with another James Street resident Robert Vanella and became its leader; he eventually managed to save enough money and opened a billiards parlor for the group, and from there grew illegal activities such as gambling and loan sharking. Torrio's business sense caught the eye of Paul Kelly, the leader of the Five Points Gang. Torrio's gang ran legitimate businesses, but its primary concern was the numbers game, supplemented by incomes from bookmaking, loan sharking, hijacking, prostitution, and opium trafficking. Al Capone, who worked at Kelly's club, admired Torrio's quick mind and looked to him as his mentor.

Capone had belonged to the Junior Forty Thieves, the Bowery Boys and the Brooklyn Rippers; they soon moved up to the Five Points Gang. One of Torrio's associates, Frankie Yale, eventually hired Capone to bartend at the Harvard Inn, a bar in the Coney Island section of Brooklyn.

By 1909, Torrio moved to Chicago. "Big Jim" Colosimo, who had become head of a burgeoning vice empire in Chicago is reputed to have invited him to the city to help him deal with Black Hand extortionists. After doing so, Torrio became a top lieutenant in Colosimo's organization, rising to underboss by 1914.

In 1919, Al Capone arrived in Chicago and started working as a bouncer and bartender at one of the Colosimo gang establishments, the Four Deuces at 2222 S. Wabash Street.

When Prohibition went into effect in 1920, Torrio pushed for the gang to enter into bootlegging, but Colosimo stubbornly refused. In March 1920, Colosimo secured an uncontested divorce from Victoria Moresco. A month later, he and Dale Winter eloped to West Baden Springs, Indiana. Upon their return, he bought a home on the South Side. On May 11, 1920, Colosimo drove to Colosimo's Cafe to meet an associate he had never met before. He was shot and killed a few minutes after entering the restaurant by a gunman hiding in the cloak room. A bullet entered Colosimo's brain, behind his right ear. Contract killer Frankie Yale had allegedly traveled from New York to Chicago and personally killed longtime gang boss Colosimo at the behest of Torrio. Although suspected by Chicago police, Yale was never officially charged. Colosimo was allegedly murdered because he stood in the way of his gang making bootlegging profits, having "gone soft" after his marriage with Winter.