RMS Dunottar Castle was a Royal Mail Ship that went into service with the Castle Line in 1890 on the passenger and mail service between Britain and South Africa. In 1913 the ship was sold to the Royal Mail Steam Packet Company as the Caribbean. After the outbreak of the First World War she served as HMS Caribbean, first as a troop ship and then as an armed merchant cruiser, until she sank in a storm off the Scottish coast on 27 September 1915.

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.