Prince Wilhelm Karl of Prussia (Wilhelm Karl Adalbert Erich Detloff; 30 January 1922, in Potsdam – 9 April 2007, in Holzminden) was the third son of Prince Oskar of Prussia, and the last surviving grandson of Wilhelm II, the last German Emperor. He was the thirty-sixth Master of Knights (Herrenmeister) of the ancient and beneficent Protestant (and largely German) Order of Saint John (Bailiwick of Brandenburg), also known as Der Johanniterorden.

Wilhelm-Karl was the youngest of Prince Oskar of Prussia and Countess Ina Marie von Bassewitz's four children. Having been admitted to the Bailiwick of Brandenburg of the Order of Saint John of the Hospital at Jerusalem (known informally as the Johanniterorden, it is the Protestant successor of the mediaeval Knights Hospitaller) in 1944, he later succeeded his father as its head, serving as the thirty-sixth Herrenmeister ("Master of the Knights") of the Order from 1958 until 1999. Prince Wilhelm-Karl worked tirelessly to keep the Order intact during the Cold War and helped to reunite its membership after the fall of East Germany.

Amongst other orders and awards, Wilhelm-Karl received the Grand Merit Cross (with star) of the Order of Merit of the Federal Republic of Germany and the Grand Cross of the Order pro merito Melitensi of the Sovereign Military Order of Malta.

In 1952, Wilhelm-Karl married Armgard von Veltheim (born 17 February 1926). The couple had two sons and a daughter:

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