United States lightship Nantucket (LV-112) is a National Historic Landmark lightship that served at the Lightship Nantucket position. She was the last serving lightship and at time of its application as a landmark, one of only two capable of moving under their own power. She served as the lightship for such notable vessels as the liners United States, Queen Mary, and Normandie.

A lightvessel, or lightship, is a ship that acts as a lighthouse. They are used in waters that are too deep or otherwise unsuitable for lighthouse construction. Although some records exist of fire beacons being placed on ships in Roman times, the first modern lightvessel was off the Nore sandbank at the mouth of the River Thames in London, England, placed there by its inventor Robert Hamblin in 1734. The type has become largely obsolete; lighthouses replaced some stations as the construction techniques for lighthouses advanced, while large, automated buoys replaced others.