USS Cyclops (AC-4) was the second of four Proteus-class colliers built for the United States Navy several years before World War I. Named after the Cyclops, a race of giants from Greek mythology, she was the second U.S. naval vessel to bear the name. The loss of the ship and 306 crew and passengers without a trace sometime after 4 March 1918 remains the single largest loss of life in the history of the United States Navy not directly involving combat.

As the loss occurred during World War I, she was thought to have been captured or sunk by a German raider or submarine because she was carrying 10,800 long tons (11,000 t) of manganese ore used to produce munitions, but German authorities at the time, and subsequently, denied any knowledge of the vessel. The Naval History & Heritage Command has stated she "probably sank in an unexpected storm", but the cause of the ship's loss is not known.