RMS Alcantara was an ocean liner of the Royal Mail Lines, a successor to the Alcantara that had been sunk in World War I.

Alcantara was built by Harland & Wolff in Belfast, and launched in 1927. She ran the route from Southampton to the east coast of South America. In 1934 the ship was rebuilt with more powerful engines, increasing her top speed to 18 knots.

She was converted to an armed merchant cruiser in 1939, and refitted with a single funnel. She was dispatched to Malta for further modifications, but was involved in a major collision with the Cunard ship Franconia en route. Alcantara managed to reach Alexandria for hull repairs.

On 28 July 1940, Alcantara encountered the German auxiliary cruiser Thor in the South Atlantic. Thor scored three hits on Alcantara, and was hit twice by Alcantara's 6-inch guns. One of the hits on Alcantara flooded the engine room, which forced her to reduce speed, allowing Thor to escape.

In 1943, Alcantara was converted into a troop ship. South American service resumed in October 1948, and lasted until April 1958, when she was sold to Japanese owners, renamed Kaisho Maru, but broken up the same year.