The Harry F. Sinclair House is a mansion at the southeast corner of East 79th Street and Fifth Avenue on the Upper East Side of Manhattan in New York City, New York, U.S. The house was built between 1897 and 1899. Over the first half of the 20th century, the house was successively the residence of businessmen Isaac D. Fletcher and Harry F. Sinclair, and then the descendants of Peter Stuyvesant, the last Director of New Netherland. The Ukrainian Institute of America acquired the home in 1955. After the house gradually fell into disrepair, the institute renovated the building in the 1990s. The house was added to the National Register of Historic Places (NRHP) and was named a National Historic Landmark in 1978.

The mansion was designed in an eclectic French Renaissance style by C. P. H. Gilbert and built by foreman Harvey Murdock. The building largely retains its original design, except for a tankhouse on the roof. Gilbert and Murdock constructed the bulk of the house with brick, which was then faced with limestone ashlar. The northern facade on 79th Street, containing the main entrance, is characterized by multiple windows in square recesses or semi-elliptical and fully Gothic arches. The western facade on Fifth Avenue is symmetrical and dominated by a curved, projecting pavilion. The interior of the mansion comprises 27 rooms on six floors, for a total floor space of 20,000 square feet (1,900 m²). Critical reviews of the house's architecture over its history have been largely positive.

The Harry F. Sinclair House is at 2 East 79th Street on the Upper East Side of Manhattan in New York City, New York, U.S. It is at the southwest corner of Fifth Avenue and 79th Street, directly across from Central Park. The Sinclair House stands on a lot measuring 100 feet (30 m) by 32.2 feet (9.8 m). The dimensions of the building itself are 96 feet (29 m), along East 79th Street, and 30 feet (9.1 m) on Fifth Avenue. The Sinclair House abuts the James B. Duke House and Payne Whitney House immediately to the south. The building is surrounded by a lawn, sunk into the ground, that is itself enclosed by a wrought iron fence, broken only by a stair and balustrade approaching the main entrance, on the north side.

The city block between Fifth Avenue, Madison Avenue, and 78th and 79th Streets was part of the Lenox family farm until 1877, when Marcellus Hartley bought the block for $420,000. The railroad magnate Henry H. Cook acquired the site for $500,000 in 1880. and owned it for the remainder of the 19th century. Cook built a house on the southwest corner of the block in 1883. Cook intended the block to house first-class residences, not high-rises, and only sold lots for the construction of private dwellings. By the early 1910s, the value of the land had increased to $6 million. Through the early 2000s, the block of Fifth Avenue remained largely intact, compared to other parts of Fifth Avenue's "Millionaire's Row".

Isaac D. Fletcher was an industrialist and art collector during the late 19th century, who was the president of the New York Coal Tar Company and the Barrett Manufacturing Company. Fletcher purchased a land lot at the corner of Fifth Avenue and 79th Street from Henry H. Cook for $200,000 (equivalent to $7,559,200 in 2024) in 1897. Fletcher, who was planning a house on the block, hired architect C. P. H. Gilbert to design the abode. At the time, Fletcher resided at the Astoria Hotel. The new house's design so impressed Fletcher that he commissioned a painting of the finished residence from Jean-François Raffaëlli in 1899. Construction was undertaken by stonemason Harvey Murdock and was completed in 1899 at a total cost of $200,000 (equivalent to $7,559,200 in 2024). After taking ownership of the house, the Fletcher family moved their large art collection there, including paintings by Jacques-Louis David, Thomas Gainsborough, Rembrandt van Rijn, Joshua Reynolds, and Peter Paul Rubens.

Fletcher died at the house in 1917, and in his will bequeathed the property to the Metropolitan Museum of Art. The museum also received the house's collection of 251 paintings. The museum sold the house the next year to oil magnate Harry F. Sinclair, who sold the house in 1930 to Augustus Stuyvesant Jr. and Anne van Horne Stuyvesant, the last direct descendants of Peter Stuyvesant, the final Dutch governor of New Netherland. The siblings resided in the mansion until their deaths in 1953 and 1938 respectively. A skylight above the staircase in the middle of the house was covered in the late 1940s.

The executors of the Stuyvesant estate sold the Sinclair House in 1954 to a group of investors, who sold it in 1955 to the Ukrainian Institute of America (UIA), a nonprofit founded by Ukrainian businessman William Dzus in 1948 to promote Ukrainian culture. The UIA's purchase of the Sinclair House gave the structure a "temporary reprieve" from demolition, as described by Newsday; at the time, several other mansions on Fifth Avenue were being demolished. The mortgage on the building was repaid in 1962.

In 1977, the New York City Landmarks Preservation Commission designated the house as part of the Metropolitan Museum Historic District, a collection of 19th- and early 20th-century mansions around Fifth Avenue between 78th and 86th Streets. That June, the American Association for State and Local History filed paperwork with the National Park Service to nominate the Sinclair House for inclusion on the National Register of Historic Places (NRHP). The next year, on June 2, 1978, it was added to the NRHP.