elector of Brandenburg (1598–1608), eldest son of Elector John George.

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. By the death of George Frederick of Prussia, Joachim 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). In Brandenburg he made concessions to the nobles at the expense of the peasantry and admitted the right of the estates to control taxation.