 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.His titles also included "duke (Dux) of Stettin, Pomerania, Cassubia, Vandalorum and Crossen", according to the terms of the Treaty of Grimnitz, although the Pomeranian titles were only nominal.