The treaty of Marienburg, concluded on 29 June 1656, was a Brandenburg-Prussian – Swedish alliance during the Second Northern War.^[1]

In January 1656, Charles X Gustav of Sweden made Frederick William I, Elector of Brandenburg, his vassal for the Duchy of Prussia.^[2] By the time of the treaty of Marienburg, Swedish prospect in the war had diminished, and Charles X Gustav was willing to offer Frederick William I a reward for fighting on his side.^[1] While the latter was to remain a Swedish vassal for Prussia, he was promised hereditary sovereignty in four voivodeships^[3] of Greater Poland in return for participating in the Charles X Gustav's Polish campaigns.^[1] This alliance proved victorious in the subsequent Battle of Warsaw, but as further campaigns stalled, Frederick William I was to gain full sovereignty in Prussia by the Treaty of Labiau in November 1656.^[4]