SS United States is a retired ocean liner built in 195051 for the United States Lines at a cost of US$79.4 million. The ship is the largest ocean liner constructed entirely in the United States and the fastest ocean liner to cross the Atlantic in either direction, retaining the Blue Riband for the highest average speed since her maiden voyage in 1952. She was designed by American naval architect William Francis Gibbs and could be converted into a troopship if required by the Navy in time of war. United States maintained an uninterrupted schedule of transatlantic passenger service until 1969 and was never used as a troopship.

The ship has been sold several times since the 1970s, with each new owner trying unsuccessfully to make the liner profitable. Eventually, the ship's fittings were sold at auction, and hazardous wastes, including asbestos panels throughout the ship, were removed, leaving her almost completely stripped by 1994. Two years later, she was towed to Pier 82 on the Delaware River, in Philadelphia, where she remains today.

Since 2009, a preservation group called the SS United States Conservancy has been raising funds to save the ship. The group purchased her in 2011 and has drawn up several unrealized plans to restore the ship, one of which included turning the ship into a multi-purpose waterfront complex. In 2015, as its funds dwindled, the group began accepting bids to scrap the ship; however, sufficient donations came in via extended fundraising. Large donations have kept the ship berthed at her Philadelphia dock while the group continues to further investigate restoration plans.

An ocean liner is a passenger ship primarily used as a form of transportation across seas or oceans. Liners may also carry cargo or mail, and may sometimes be used for other purposes (such as for pleasure cruises or as hospital ships).Cargo vessels running to a schedule are sometimes called liners. The category does not include ferries or other vessels engaged in short-sea trading, nor dedicated cruise ships where the voyage itself, and not transportation, is the prime purpose of the trip. Nor does it include tramp steamers, even those equipped to handle limited numbers of passengers. Some shipping companies refer to themselves as "lines" and their container ships, which often operate over set routes according to established schedules, as "liners".

Ocean liners are usually strongly built with a high freeboard to withstand rough seas and adverse conditions encountered in the open ocean. Additionally, they are often designed with thicker hull plating than is found on cruise ships, and have large capacities for fuel, food and other consumables on long voyages.The first ocean liners were built in the mid-19th century. Technological innovations such as the steam engine and steel hull allowed larger and faster liners to be built, giving rise to a competition between world powers of the time, especially between the United Kingdom and Germany. Once the dominant form of travel between continents, ocean liners were rendered largely obsolete by the emergence of long-distance aircraft after World War II. Advances in automobile and railway technology also played a role. After Queen Elizabeth 2 was retired in 2008, the only ship still in service as an ocean liner is RMS Queen Mary 2.