Alcyon was a French fishing trawler that was seized in the Second World War by the Kriegsmarine for use as a harbour defence boat and later a vorpostenboot, serving as Boot 10 Alcyon and V 420 Alcyon. She was bombed and sunk in the Gironde on 3 August 1943.

Alcyon was 40.74 metres (133 ft 8 in) long, with a beam of 6.60 metres (21 ft 8 in).^[1] She had a depth of 2.87 metres (9 ft 5 in),^[2] and a draught of 3.56 metres (11 ft 8 in).^[1] She was assessed at 266 GRT, 103 NRT. She was powered by a triple expansion steam engine, which had cylinders of 33 centimetres (13 in), 55 centimetres (21+1⁄2 in) and 89 centimetres (35 in) diameter by 61 centimetres (24 in) stroke. The engine was made by Alblasserdamsche Machienfabriek, Alblasserdam, South Holland, Netherlands. It was rated at 60nhp. The engine powered a single screw propeller.^[2] It could propel the ship at 10 knots (19 km/h).^[1]

Alcyon was built as yard number 163 by Bonn & Mees, Rotterdam, South Holland in 1904 for French owners.^[1] The Code Letters OBNH were allocated,^[2] as was the fishing boat registration B 2892.^[1] By 1930, she was owned by Bourgain-Vincent, Boulogne.^[2] In 1934, her Code Letters were changed to FNOH. She was sold to E. Malfoy & Fils in that year.^[3]

On 26 June 1940, Alcyon was seized by the Germans at La Rochelle, Charente-Inférieure. She was allocated to the Hafenschutz-Flotille Lorient on 2 November, serving as Boot 10 Alcyon. On 4 May 1942, Alcyon was redesignated as a vorpostenboot. She was allocated to 4 Vorpostenflotille as V 420 Alcyon. On 2 August 1943, she was sunk in the Gironde in an attack by British aircraft with the loss of five of her crew.^[4][5]