Thornwood is a hamlet (unincorporated community), census-designated place (CDP), and postal designation (with zip code 10594) in the town of Mount Pleasant in Westchester County, New York.

The village was dissolved in 1914, when Thornwood took its current name and became a hamlet within the Town of Mount Pleasant.

Thornwood once had a large and thriving Westchester marble quarry near its heart, the intersection of Route 141 and Kensico Road (known as Four Corners).

The quarry pit was filled in during the mid-1980s, and the Town Center shopping mall was constructed over it.

Thornwood once had a station on the Harlem Line of the Metro-North Railroad and was approximately a 48-minute ride to Grand Central Terminal.

Thornwood is the home to the North American headquarters for the Carl Zeiss Corporation, a multinational German manufacturer of optical systems, industrial measurements and medical devices.

The Legion of Christ, a Roman Catholic religious institute, operates the Our Lady of Thornwood Conference, Education and Training Center on the site of a former IBM facility on Columbus Avenue.

The seminarians studying here for the Catholic priesthood usually major in philosophy at the Thornwood Conference Center.

However, due to a lack of vocations and private funds—partly due to the abuse scandal caused by the order's founder, Father Marcial Maciel Degollado, which resulted in a Vatican investigation and revision—the entire course of Seminary studies will now be taken at the Legionaries' Seminary in Rome, Italy and the building will be sold.