Sag Harbor is an incorporated village in Suffolk County, New York, United States. The village developed as a working port on Gardiner’s Bay. The entire business district is listed as the historic Sag Harbor Village District on the National Register of Historic Places. A major whaling and shipping port in the 19th century, by the end of this period and in the 20th century, it became a destination for wealthy people who summered here. Some writers and artists also settled here. Sag Harbor is about three fifths in Southampton and two fifths in East Hampton. Its landmarks include structures associated with whaling and its early days when it was designated as the first port of entry to the new United States. It had the first United States custom house erected on Long Island.