The first USS Gettysburg was a steamer in the United States Navy. The ship was built in Glasgow, Scotland, in 1858, named RMS Douglas, and operated by the Isle of Man Steam Packet Company in the United Kingdom between Liverpool, England, and Douglas on the Isle of Man until November 1862. She was then sold to Cunard, Wilson & Company on behalf of the Confederate agents Fraser, Trenholm & Company for use by the Confederate States of America during the American Civil War. Renamed Margaret and Jessie, she operated as a blockade runner until her capture by the Union on 5 November 1863. The ship then was commissioned into the Union Navy on 2 May 1864 as USS Gettysburg.

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.