HELVOETSLUYS (in Dutch, Hellevoetsluis'), a fortified town of the Netherlands, in the province of South Holland, situated in the south of the island of Voorne-and-Putten, on the shore of the Haringvliet. It possesses a good harbour, a dry and a wet dock, extensive wharves, and a naval arsenal, and among its more important public buildings are the communal chambers, the Reformed church dating from 1661—1684, the Roman Catholic church, the synagogue, and the naval college. The population was only 1208 in 1795; by 1840 it had increased to 2523, and by 1874 it was 4135.

Helvoetsluys was founded in the 17th century. The construction of the dock was decreed by the states of Holland in 1636, and the erection of the fortifications in 1638; but it was not till 1696 that the works were completed. The new harbour soon became an important rendezvous for the Dutch fleet, and iu 1688 it was the starting-point of William of Orange’s expedition to England.