Frederick William, Duke of Brunswick-Wolfenbüttel (German: Friedrich Wilhelm; 9 October 1771 – 16 June 1815) was a German prince and Duke of Brunswick-Lüneburg and Oels. Nicknamed "The Black Duke", he was a military officer who led the Black Brunswickers against French domination in Germany. He briefly ruled the state of Brunswick-Wolfenbüttel from 1806 to 1807 and again from 1813 to 1815.

Prince Frederick William of Brunswick-Wolfenbüttel was born in Braunschweig as the fourth son of Charles William Ferdinand, Duke of Brunswick-Lüneburg, and Princess Augusta of Great Britain. He was the cousin and brother-in-law (from 8 April 1795) of his friend George IV, Prince Regent of the United Kingdom (from 1811).

He joined the Prussian army in 1789 as a captain and participated in battles against Revolutionary France. In 1805, after his uncle, Frederick Augustus, Duke of Oels, had died childless, Frederick William inherited the Duchy of Oels, a small mediatized principality in Silesia subordinate to the King of Prussia.

In October 1806, Frederick William participated in the Battle of Jena-Auerstädt as a major general of the Prussian army, of which his father was the field marshal. His father died from a wound he received in this battle, and Frederick William inherited Brunswick-Wolfenbüttel, since his eldest brother had died childless two months earlier, and both the second and third brother were mentally retarded. After the defeat of Prussia in the Fourth Coalition, his state remained under the control of France, however, and was formally made a part of the short-lived Napoleonic Kingdom of Westphalia in 1807. Frederick William fled to his parents-in-law in Bruchsal in the Grand Duchy of Baden, which had remained a sovereign state with the dissolution of the Holy Roman Empire in 1806 by Francis II, where he lived for the next few years.

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