William Frederick (Dutch: Willem Frederik) was Count (from 1654 Imperial Prince) of Nassau-Dietz, Stadtholder of Friesland, Groningen and Drenthe.
He was the second son of Ernest Casimir I, Count of Nassau-Dietz and Sophia Hedwig of Brunswick-Lüneburg. He married Albertine Agnes of Nassau, the fifth daughter of Frederick Henry, Prince of Orange on 2 May 1652 in Cleves.
As a second son, William Frederick did not seem destined for the career he eventually would follow. He studied at Leiden University and the University of Groningen and subsequently took a commission in the army of the Dutch Republic, like his male ancestors and his brother. As such he was a junior partner of his future father in law and brother in law William II, Prince of Orange. However, his elder brother died in action near Hulst in 1640. As Henry Casimir was unmarried, and did not have children, William Frederick inherited his titles.
However, as the office of stadtholder was not yet hereditary, William Frederick only managed to be appointed in Friesland. The stadtholdership in Groningen and Drenthe went to Frederick Henry, not without a struggle with William Frederick, however. After Frederick Henry's death in 1647 William II succeeded his father also in these two provinces as stadtholder. Only when William II died in 1650, just a week before his son William III was born, did William Frederick obtain the stadtholdership in the other two provinces also. At that time he might have obtained the stadtholdership in the five other provinces (Holland, Zeeland, Utrecht, Gelderland and Overijssel) also. After all, the stadtholderate was an appointive office. The elder branch of the Nassau family might have "first claim" to the office, but as the "claimant" was a newborn babe (William III), such a claim was not to be taken seriously. Yet, to avoid a quarrel with the members of that elder branch (William II's widow and mother) William Frederick did not press his personal claim.
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. The States kept their word, accepting a "regency" of the young boy's mother. The Frisian stadtholderate was made hereditary in 1675.