The Battle of the Raz de Sein was a single-ship naval engagement of the blockade of Brest during the French Revolutionary Wars between a French and Royal Navy ships of the line on 21 April 1798. The British blockade fleet under Admiral Lord Bridport had sailed from St Helens on 12 April and on the morning of 21 April was crossing the Iroise Passage when sails were spotted to the east. Three ships were detached in pursuit, led by the 74-gun ship of the line HMS Mars under Captain Alexander Hood. As the British ships approached their quarry a third sail was sighted to the southeast close to the coastline and moving north towards Brest.

Captain Alexander Hood was an officer of the Royal Navy, one of several members of the Hood family to serve at sea, including his brother Sir Samuel Hood, who were both sponsored into the Royal Navy by their cousins once removed, Viscount Hood and Alexander Hood.