SS United States is a retired American ocean liner that was built during 1950 and 1951 for United States Lines. She is the largest ocean liner to be entirely constructed in the United States and the fastest ocean liner to cross the Atlantic Ocean in either direction, earning the Blue Riband for the highest average speed since her maiden voyage in 1952, a title that remains uncontested.

The ship was designed by American naval architect William Francis Gibbs and could have been converted into a troopship if required by the United States Navy in time of war. The ship served as a US icon, transporting celebrities and immigrants throughout her career between 1952 and 1969. Her design included innovations in steam propulsion, hull form, fire safety, and damage control. Despite her record speed, passenger counts declined in the mid-1960s due to the rise in jet-propelled trans-Atlantic flights.

Following the financial collapse of United States Lines, United States was withdrawn from service in a surprise announcement in 1969. All planned cruises were canceled, and the ship changed owners repeatedly for the next several decades. Every owner attempted to make the ship profitable, but she was aging and poorly maintained. In 1984, her interior furnishings were sold at auction, and the rest of her interiors were stripped to the bulkheads in 1994. In 1996, she was towed to Philadelphia, where she remained until February 2025.

Since 2009, the SS United States Conservancy has been raising funds in an attempt to save the ship from being scrapped. The group purchased her in 2011 and has created several unrealized plans to restore the ship. Due to a rent dispute, in 2024, the ship was evicted from her pier. Because no other locations for the ship could be found, Okaloosa County, Florida, bought her and plans to sink her by 2026 near Destin to become the world's largest artificial reef.

SS United States was designed by William Gibbs. When Gibbs was eight years old in 1894, he watched the launching of SS St. Louis and became enamored with ships and dreamed of a massive, grand American ship to surpass all others.

As an adult, Gibbs' first project was to lead the redesign and reconstruction of the ocean liner Leviathan, the largest ship in the world and an American war prize following World War I. Once complete, his skill was appreciated by the government, media, and other architects; groups such as the Pacific Marine Review referred to him as "America's foremost naval architect".

The first ship purely of Gibb's design was SS Malolo, a luxury liner for the Pacific Ocean. During her sea trials, the ship was struck amidship by a freighter, ripping a massive hole into Malolo's engine room. The damage was so severe the former Chief of US Navy Construction and Gibb's mentor David Taylor thought the ship would immediately sink, much like the RMS Empress of Ireland did in similar circumstances. The ship stayed afloat with little damage and few casualties; this success was credited to Gibb's meticulous designs, further increasing his fame.

Gibbs' work put him in contact with American officials, to whom he pitched his idea for a massive, American-flagged liner. By 1936, the US government was planning to replace the aging Leviathan so a new vessel could operate as a passenger liner during peacetime and as a troop transport during war. Although not the massive ocean liner Gibbs had envisioned, he was selected to design the new ship, which became the SS America.