The Tweed Courthouse (also known as the Old New York County Courthouse) is a historic courthouse building at 52 Chambers Street in the Civic Center of Manhattan in New York City, New York, U.S. It was built in the Italianate style with Romanesque Revival interiors. William M. "Boss" Tweed – the corrupt leader of Tammany Hall, a political machine that controlled the New York state and city governments when the courthouse was built – oversaw the building's erection. The Tweed Courthouse served as a judicial building for New York County, a county of New York state coextensive with the New York City borough of Manhattan. It is the second-oldest city government building in the borough, after City Hall.

The structure comprises pavilions to the east and west of a central section, as well as a rear wing to the south. Architect John Kellum and political appointee Thomas Little designed the first portion of the building, which was constructed from 1861 to 1872. Construction was interrupted in 1871 when Kellum died and the corruption involved in the building's construction was exposed to the public. The project was completed by architect Leopold Eidlitz, who added the rear wing and finished the interior between 1877 and 1881.

The media criticized the project as wasteful and gaudy during the courthouse's construction, and for a century after its completion, there were frequent proposals to demolish the building. Several modifications were made after completion, including removal of its front steps. Modern restoration and historic preservation were completed in 2001. The building has since housed the New York City Department of Education's headquarters on its upper floors and schools on its ground level. The Tweed Courthouse is listed on the National Register of Historic Places as a National Historic Landmark, and its facade and interior are both New York City designated landmarks.

The Tweed Courthouse is in the Civic Center neighborhood of Manhattan in New York City, New York, U.S., within the northern portion of City Hall Park and just north of New York City Hall. The plot is bounded by Chambers Street to the north, Centre Street to the east, and Broadway to the west. The structure measures 258 by 149 feet (79 by 45 m), the longer side being located on the west–east axis. The portion of the park outside the courthouse building contains shaded walkways and lawns. Across Chambers Street, the Tweed Courthouse faces 280 Broadway, 49 Chambers, and the Surrogate's Courthouse from west to east. The Manhattan Municipal Building and the Brooklyn Bridge ramp are across Centre Street. Several buildings face the Tweed Courthouse on Broadway, including the Broadway–Chambers Building, Tower 270, the Rogers Peet Building, and the Home Life Building.

The Tweed Courthouse is the second-oldest municipal government building in Manhattan, after New York City Hall. During the 17th century, the site had been occupied by the city's public commons. The Dutch settlers of New Amsterdam used the location as a grazing site for animals. In 1686, the courthouse site was acquired by the English authorities as a punishment location for prisoners and an African burial ground. Eight graves from the American colonial era still exist beneath the courthouse. Other government buildings were also built, including an almshouse, the Upper Barracks, the New Gaol, a military jail called the Bridewell, and a second almshouse. The now-demolished Rotunda art gallery was directly to the east of the courthouse's site.

The Tweed Courthouse includes a central section, two wings on the western and eastern ends, and an annex on its southern portion. It is four and a half stories tall. The floor count includes a half-story attic, but not the building's two mezzanine levels, which are considered to be intermediate staircase landings. The first floor is at ground level but was formerly known as the basement. The structure lies atop a low foundation made of granite. The roof was replaced three times: first with iron in the early 20th century, then with asphalt in 1978 or 1979, and finally with a stainless steel-over-rubber surface in 2001. The Guide to New York City Landmarks characterizes the building as containing "some of the finest mid-19th century interiors in New York".

John Kellum and Thomas Little were responsible for the courthouse's initial design. Kellum was hired for the Tweed Courthouse project in August 1861 and died exactly ten years later. While his obituary in Harper's Weekly praised him profusely, an anonymous writer for the American Architect and Building News said his involvement in the Tweed Courthouse negated the merits of anything else he had designed. Thomas Little, a political appointee of the New York City Board of Supervisors, was given ex officio credit by virtue of his membership on the Board of Supervisors. He submitted plans for the Tweed Courthouse in 1859; documents and testimony indicate that Little was likely the first architect of the courthouse. Leopold Eidlitz, who was hired to finish the courthouse in 1876, added the building's south wing and domed rotunda in a similar design to the New York State Capitol. The Romanesque style and his extensive use of brick and stone contrasted with Kellum's intricate cast-iron design. The New York City Landmarks Preservation Commission (LPC) said Eidlitz's style departs from Kellum's classicism with "an American version of organic architecture expressed through medieval forms".

The two original wings, designed by Kellum, were arranged in an Ɪ shape. The wings measured 250 feet (76 m) wide along Chambers Street and 150 feet (46 m) deep. The entry portico on the main Chambers Street elevation of the facade rises three and a half stories from a low granite curb. Panels of granite, Tuckahoe marble, and Sheffield marble are anchored on the facade, with rusticated stone at the basement level. Behind the granite and marble cladding is the brick superstructure. According to an LPC report in 1984, no documentation regarding the use of other quarries was found.