The Chicago Outfit, also known as the Outfit, the Chicago Mafia, the Chicago Mob, the Chicago crime family, the South Side Gang or the Organization, is an Italian American Mafia crime family based in Chicago, Illinois, and throughout the Greater Chicago area, originating in the city's South Side in the early 1910s.

The Outfit rose to power in the 1920s under the control of Johnny Torrio and Al Capone, and the period was marked by bloody gang wars for control of the distribution of illegal alcohol during Prohibition. The Outfit's power was solidified by Capone's leadership, consolidating the family into the larger American Mafia. Since then, the Outfit has been involved in a wide range of criminal activities, including loansharking, illegal gambling, prostitution, extortion, political corruption and murder. Capone was convicted of income tax evasion in 1931 and the Outfit was next run by Paul Ricca. Ricca and Tony Accardo shared power from 1943 until Ricca's death in 1972; Accardo became the sole power in the Outfit upon Ricca's death and was one of the longest-sitting bosses of all time upon his death in 1992. The family’s longest-serving boss was Joey Aiuppa, serving from 1971 until 1986.

Although it has never had a complete monopoly on organized crime in Chicago, the Outfit has long been the largest, most powerful and most violent criminal organization in Chicago and the Midwest in general. Unlike other Mafia factions such as the Five Families of New York City, the Outfit has been a unified faction since its conception. Its influence at its peak stretched as far as California, Florida and Nevada and it continues to operate throughout the Midwestern United States and South Florida, as well as Las Vegas and other parts of the Southwestern United States. Heightened law enforcement attention and general attrition have led to its gradual decline since the late 20th century, though it continues to be one of the major and most active organized crime groups in the Chicago metropolitan area and the Midwest.

The early years of organized crime in Chicago, in the late 19th and early 20th centuries, were marked by the division of various street gangs controlling the South Side and North Side, as well as the Black Hand organizations of Little Italy. In later years, the Outfit consisted of various street crews controlling different territories around Chicago including Elmwood Park, Melrose Park, Chicago Heights, Rush Street, Grand Avenue and Chinatown.

Vincenzo "Big Jim" Colosimo centralized control in the early 20th century. Colosimo was born in Calabria, Italy, in 1878, and immigrated to Chicago in 1895, where he established himself as a criminal. By 1909, with the help of bringing Johnny Torrio from New York to Chicago, he was successful enough that he was encroaching on the criminal activity of the Black Hand organization. Colosimo also "cultivated deep political connections" after "serving as a precinct captain in the organization of First Ward Alderman Couglin and Kenna, and later [became] the bagman (collector of illegal profits and dispenser of bribes) in the vice-laden Levee District, which afforded him with blanket political protection".

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 his wife, Victoria Moresco. A month later, he and singer Dale Winter eloped to West Baden Springs, Indiana. Upon their return, he bought a home on the South Side. On May 11, 1920, Colosimo was killed by a gunman waiting in the coat room of his restaurant, Colosimo's cafe. The killer was most likely Frankie Yale, fulfilling a contract commissioned by Johnny Torrio.

Torrio's organization was made up predominately of ethnic Italians but had a large contingent of members from other immigrant backgrounds. Torrio's gang also differed from the other Chicago gangs by recruiting from New York's underworld, regularly welcoming that city's ambitious criminals into his organization's ranks. Alphonse "Scarface Al" Capone had left New York for Chicago in 1919, likely under orders from mob boss Frankie Yale to leave town to avoid retaliation for previous violence they had committed in New York. Capone began in Chicago as a doorman at the Four Deuces club. By 1924, Capone's business acumen and shrewd intelligence had gained him a place as Torrio's right-hand man. Many rivals saw Capone as responsible for the proliferation of brothels in the city.

In 1923, William Dever was elected mayor of Chicago, and he began to crack down on Torrio's underworld activities within the city. Losing their political cover, the gang looked to the suburbs for a new base of operations. Torrio ordered Capone to lead a takeover of the town of Cicero, where he was able to corrupt city manager Joseph Z. Klenha. The gang set up dozens of brothels, speakeasies, gambling dens, and other businesses within the town. On April 1, 1924, Cicero municipal elections were threatening Kenha's leadership. Kenha appealed to Torrio and Capone for their support, which they responded to by terrorizing the opposition and voting public on election day. The gang guarded polling places, ensuring people knew the right way to vote and violently preventing entry to those who did not. They ransacked the local Democratic party campaign headquarters, forcibly detaining the election workers for hours. By that afternoon, the Chicago Police Department was ordered to step in to halt the violence by Cook County Judge Edmund J. Jareki. Seventy plainclothes officers, newly deputized as county sheriffs, descended on Cicero. Frank Capone, Al's brother, was killed that evening by detective sergeant William Cusack during an altercation on 22nd Street and Cicero Avenue. Charles Fischetti was also arrested at the scene. Kenha won the election, ensuring the Torrio-Capone gang's local immunity into the 1930s.