Willem Bloys van Treslong was a nobleman from the Southern Netherlands and military leader during the Dutch war of Independence. He was best known as one of the leaders of the Sea Beggars who captured Den Briel on 1 April 1572.
The family Bloys van Treslong had lands in Flanders, Hainaut and Holland; they are descended from John of Beaumont. His was the bailiff of Voorne.
Bloys van Treslong left Spanish service in 1558, and in 1567, joined other high nobles of the Netherlands in refusing to pledge allegiance to Margaret of Parma, the governor of the Netherlands, and was part of the Compromise of Nobles. He fought in the battle of Heiligerlee in 1568. In 1571, William the Silent provided him with letters of marque and equipped two ships to join the Sea Beggars. In March 1572, Bloys van Treslong's ships were trapped by ice at Wieringen and are attacked by four Spanish companies of infantry. Bloys escaped from the Spaniards but lost his sword which at present hangs in the Michaëlskerk church in Oosterland. In April 1572, the Sea Beggars captured Briel, which became a rebel base. Later, in 1572, the rebels captured Flushing.