HMT Aragon, originally RMS Aragon, was a 9,588 GRT transatlantic Royal Mail Ship that served as a troop ship in the First World War. She was built in Belfast, Ireland in 1905 and was the first of the Royal Mail Steam Packet Company's fleet of "A-liners" that worked regular routes between Southampton and South American ports including Buenos Aires.

Royal Mail Ship, usually seen in its abbreviated form RMS, is the ship prefix used for seagoing vessels that carry mail under contract to the British Royal Mail. The designation dates back to 1840. Any vessel designated as "RMS" has the right both to fly the pennant of the Royal Mail when sailing and to include the Royal Mail "crown" insignia with any identifying device and/or design for the ship.