William Louis of Nassau-Dillenburg (Dutch: Willem Lodewijk; West Frisian: Willem Loadewyk; 13 March 1560, Dillenburg, Hesse – 13 July 1620, Leeuwarden, Netherlands) was Count of Nassau-Dillenburg from 1606 to 1620, and stadtholder of Friesland, Groningen, and Drenthe.

William Louis was the eldest son of John VI, Count of Nassau-Dillenburg and his first wife, Elisabeth of Leuchtenberg.

In a letter to his cousin Maurice of Nassau, Prince of Orange which he composed on 8 December 1594, he set out (from reading Aelianus Tacticus) an argument based on the use of ranks by soldiers of Imperial Rome as discussed in Aelian's Tactica.

Aelian was discussing the use of the counter march in the context of the Roman sword gladius and spear pilium.

He died in his home, the Stadhouderlijk hof in Leeuwarden, the city which honored him with a statue on the government square.