RV Calypso is a former British Royal Navy minesweeper converted into a research vessel for the oceanographic researcher Jacques Cousteau, equipped with a mobile laboratory for underwater field research. She was severely damaged in 1996 and was planned to undergo a complete refurbishment in 2009–2011 that has not been accomplished.^{[needs update]} The ship is named after the Greek mythological figure Calypso.

Calypso was originally a minesweeper built by the Ballard Marine Railway Company of Seattle, Washington, United States for the United States Navy for loan to the British Royal Navy under lend-lease. A wooden-hulled vessel, built of Oregon pine.

Originally constructed as a British yard minesweeper (BYMS) Mark 1 class motor minesweeper, laid down on 12 August 1941 with yard designation BYMS-26 and launched on 21 March 1942, and was commissioned into the Royal Navy in February 1943 as HMS J-826 and assigned to active service in the Mediterranean Sea, based in Malta, and was reclassified as BYMS-2026 in 1944. Following the end of World War II, she was decommissioned in July 1946 and laid up at Malta. On 1 August 1947 the vessel was formally handed back to the US Navy and then struck from the US Naval Register, remaining in lay-up.

In May 1949 the vessel was bought by Joseph Gasan of Malta, who had secured the mail contract on the ferry route between Marfa, in the north of Malta, and Mġarr, Gozo, in the south of Malta, in 1947. She was converted to a ferry and renamed Calypso G after the nymph Calypso, whose island of Ogygia was mythically associated with Gozo, entering service in March 1950. After only four months on the route, Gasan sold her to new owners.

The British millionaire and former Member of Parliament (MP), Thomas Loel Guinness bought Calypso in July 1950. He leased her to Cousteau for a symbolic one franc a year. He had two conditions: that Cousteau never ask him for money and that he never reveal his identity, which only came out after Cousteau's death. Cousteau restructured and transformed the ship into an expedition vessel and support base for diving, filming and oceanographic research. One of the more unusual expeditions involving the vessel was a survey of Abu Dhabi waters conducted by Cousteau on behalf of British Petroleum (BP) in 1954 – the first and last time it was used for an oil survey.

Calypso carried advanced equipment, including one- and two-man mini submarines developed by Cousteau, diving saucers, and underwater scooters. The ship was also fitted with a see-through "nose" and an observation chamber 3 metres (9.8 ft) below the waterline, and was modified to house scientific equipment and a helicopter pad. The Calypso underwater camera is named after this ship.

On 8 January 1996, a barge accidentally rammed Calypso and sank her in the port of Singapore. On 16 January, she was raised by a 230-foot (70 m) crane, patched, and pumped dry before being put in a shipyard.

Calypso was later towed to the Maritime Museum of La Rochelle in 1998 to be an exhibit.