William Magear Tweed (April 3, 1823 – April 12, 1878), often erroneously referred to as William "Marcy" Tweed (see below), and widely known as "Boss" Tweed, was an American politician most notable for being the political boss of Tammany Hall, the Democratic Party's political machine that played a major role in the politics of 19th-century New York City and state. At the height of his influence, Tweed was the third-largest landowner in New York City, a director of the Erie Railroad, a director of the Tenth National Bank, a director of the New-York Printing Company, the proprietor of the Metropolitan Hotel, a significant stockholder in iron mines and gas companies, a board member of the Harlem Gas Light Company, a board member of the Third Avenue Railway Company, a board member of the Brooklyn Bridge Company, and the president of the Guardian Savings Bank.

Tweed was elected to the United States House of Representatives in 1852 and the New York County Board of Supervisors in 1858, the year that he became the head of the Tammany Hall political machine. He was also elected to the New York State Senate in 1867. However, Tweed's greatest influence came from being an appointed member of a number of boards and commissions, his control over political patronage in New York City through Tammany, and his ability to ensure the loyalty of voters through jobs he could create and dispense on city-related projects.

Tweed was convicted for stealing an amount estimated by an aldermen's committee in 1877 at between $25 million and $45 million from New York City taxpayers from political corruption, but later estimates ranged as high as $200 million. Unable to make bail, he escaped from jail once but was returned to custody. He died in the Ludlow Street Jail.