Tammany Hall, also known as the Society of St. Tammany, the Sons of St. Tammany, or the Columbian Order, was an American political organization founded in 1786 and incorporated on May 12, 1789, as the Tammany Society. It became the main local political machine of the Democratic Party and played a major role in controlling New York City and New York State politics, and helped immigrants, most notably the Irish, rise in American politics into the 1960s. Tammany typically controlled Democratic nominations and political patronage in Manhattan after the mayoral victory of Fernando Wood in 1854, and used its patronage resources to build a loyal, well-rewarded core of district and precinct leaders; after 1850 the vast majority were Irish Catholics due to mass immigration from Ireland during and after the Irish Famine.

Tammany emerged as the center of Democratic-Republican Party politics in the city in the early 19th century. After 1854, it expanded its political control even further by earning the loyalty of the city's rapidly expanding immigrant community, which functioned as its base of political capital. The business community appreciated its readiness, at moderate cost, to cut through regulatory and legislative mazes to facilitate rapid economic growth.

By 1872, Tammany had an Irish Catholic "boss", and in 1928 a Tammany hero, New York Governor Al Smith, won the Democratic presidential nomination. However, the organization also served as an engine for graft and political corruption, perhaps most infamously under William M. "Boss" Tweed in the mid-19th century. The Tammany ward boss or ward heeler, as wards were the city's smallest political units from 1786 to 1938, served as the local vote gatherer and provider of patronage.

By the 1880s, Tammany was building local clubs that appealed to social activists from the ethnic middle class. At its peak the machine had the advantage of a core of solid supporters, and usually exercised control of politics and policymaking in Manhattan; it also played a major role in the state legislature in Albany.

Charles Murphy acted as boss from 1902 to 1924. "Big Tim" Sullivan was the Tammany leader in the Bowery, and the machine's spokesman in the state legislature. In the early 20th century the two men promoted Tammany as a reformed agency dedicated to the interests of the working class. The new image deflected attacks and secured a following among the emerging ethnic middle class. In the process Robert F. Wagner became a powerful United States Senator, and Smith served four terms as governor and was the Democratic presidential nominee in 1928.

Tammany's influence waned during the 1930s and early 1940s, when it engaged in a losing battle with Franklin D. Roosevelt, the state's governor (1929–1932) and later U.S. President (1933–1945). In 1932, after Mayor Jimmy Walker was forced from office when his bribery was exposed, Roosevelt stripped Tammany of federal patronage. Republican Fiorello La Guardia was elected mayor on a Fusion ticket and became the first anti-Tammany mayor to be re-elected. A brief resurgence in Tammany power in the 1950s under the leadership of Carmine DeSapio was met with Democratic opposition led by Eleanor Roosevelt, Herbert Lehman, and the New York Committee for Democratic Voters. By the mid-1960s Tammany Hall ceased to exist.