Joachim Frederick (German: Joachim Friedrich) (27 January 1546 – 18 July 1608), of the House of Hohenzollern, was Prince-elector of the Margraviate of Brandenburg from 1598 until his death. Joachim Frederick was born in Cölln to John George (1525-1598), Elector of Brandenburg, and Sophie of Legnica (1525-1546). He served as administrator of the Archbishopric of Magdeburg from 1566 to 1598 and then succeeded his father as Elector of Brandenburg in 1598. Joachim Frederick's first marriage on 7 March 1570 was to Catherine of Brandenburg-Küstrin, daughter of John, Margrave of Brandenburg-Küstrin, and Catherine of Brunswick-Wolfenbüttel. Joachim Frederick's second marriage, on 23 October 1603, was to Eleanor of Prussia, born 12 August 1583, daughter of Albert Frederick and Marie Eleonore of Cleves. He became regent of the Duchy of Prussia in 1605. By the death of George Frederick of Prussia, Joachim Frederick became regent of the duchy of Prussia (ruled nominally by the mentally retarded Albert Frederick), but he had some difficulty in asserting his position (the position being established more firmly by his son and heir John Sigismund, who eventually became duke of Prussia). Joachim established the rule of primogeniture for the Hohenzollern electorate by a family agreement known as the Gera Bond (1598), which confirmed the practice begun by Albert III Achilles whereby Brandenburg formed the inheritance of the elector’s eldest son. In Brandenburg he made concessions to the nobles at the expense of the peasantry and admitted the right of the estates to control taxation.

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