Sluis (slois), municipality, Zeeland prov., SW Netherlands, on the Scheldt estuary, near the Belgian border. Sluis was founded in the 13th cent. and later accorded trading privileges to the Hanseatic League. In 1340, Edward III of England defeated the fleet of Philip VI of France off nearby Terneuzen in the first important engagement of the Hundred Years War. Sluis fell to the Spanish in 1587 and was recovered by the Dutch in 1604. It subsequently lost importance as a port. It is also known by its French name, L'Écluse. The Dutch name was formerly spelled Sluys.