William Frederick (Dutch: Willem Frederik; Arnhem 7 August 1613 – Leeuwarden 31 October 1664), was Count (from 1654 Imperial Prince) of Nassau-Dietz, Stadtholder of Friesland, Groningen and Drenthe.

He married Countess Albertine Agnes of Nassau, the fifth daughter of Frederick Henry, Prince of Orange on 2 May 1652 in Cleves.

When John died in 1606 his inheritance was divided among his five sons, one of which was William Frederick's father Ernest Casimir, who received the title of Count of Nassau-Dietz and followed his eldest brother William Louis, Count of Nassau-Dillenburg as Stadtholder of Friesland, Groningen and Drenthe in 1620.

William Frederick had played a key role in that coup by leading the attempt to seize the city of Amsterdam by force in August, 1650.

[4] However, after William's death (Nov 1650) the Holland Regents seized their chance to revert to the status quo ante.

On the federal level William II had fulfilled the office of Captain general of the Union, like his father and uncle before him.

[7] Only during the campaign against Bernhard von Galen during the Münster occupation of East Friesland, was he entrusted with a command in the field.

He was successful in the reconquest of a strategic fortress (the Deilerschans), but shortly afterward he died on 31 October 1664 in an accident with a pistol that fired unexpectedly.

Before his death he had persuaded the States of Friesland that his son Henry Casimir II (only 7 years old in 1664) should succeed him as stadtholder.