The Gangs of New York: An Informal History of the Underworld is an American non-fiction book by Herbert Asbury, first published in 1928 by Alfred A. Knopf. It formed the basis for Martin Scorsese's 2002 Academy Award-winning film Gangs of New York, starring Leonardo DiCaprio, Daniel Day-Lewis, and Cameron Diaz.

In 1927, Asbury published an article in The American Mercury, titled "The Old-Time Gangs of New York", which was ultimately incorporated into the book published the following year. Drawing on the muckraking style of the era, the author wrote based on "legend, memory, police records, the self-aggrandizements of aging crooks, popular journalism, and solid historical research", to present an illuminating account of the gangs of old New York that ultimately gave rise to the Mafia as we know it today, from films like The Godfather.

The book outlines the rise and fall of 19th century gangs in New York City, prior to the domination of the Italian-American Mafia during Prohibition in the 1920s. Focusing on the saloon halls, gambling dens, and winding alleys of the Bowery and the Five Points district of Lower Manhattan, the book evokes the destitution and violence of a turbulent era, when colorfully named criminals like "Dandy" Johnny Dolan, William Poole (also known as "Bill the Butcher"), and Hell-Cat Maggie lurked in the shadows, corrupt politicians like William "Boss" Tweed run the city, and infamous gangs including the Plug Uglies, Dead Rabbits, and Bowery Boys ruled the streets. It includes a rogues' gallery of prostitutes, pimps, poisoners, pickpockets, murderers, and thieves.

The book contains detailed accounts of the New York City Draft Riots of 1863. It also elaborates on numerous other criminal influences of the time, including river pirates and the corrupt political establishment, such as Tammany Hall.