HELVOETSILIUS, or HELLEVOETSILIUS, a strongly fortified seaport-town of Holland, province of South Holland, on the right bank of the River Flakkee or Haringvliet, the largest mouth of the Rhine, 16 miles S.W. of Rotterdam. It has an excellent harbour and large naval dockyard. Pop. about 2000. William III. embarked here for England 11th Nov. 1688.