The Treaty of Bromberg (German: Vertrag von Bromberg, Latin: Pacta Bydgostensia) or Treaty of Bydgoszcz was a treaty between John II Casimir Vasa and Elector Frederick William of Brandenburg-Prussia that was ratified at Bromberg (Bydgoszcz) on 6 November 1657. The treaty had several agreements, including the Treaty of Wehlau, signed on 19 September 1657 by the Brandenburg–Prussian and Polish–Lithuanian envoys in Wehlau (Welawa, now Znamensk). Thus, the Treaty of Bromberg is sometimes referred to as treaty of Wehlau-Bromberg or Treaty of Wehlau and Bromberg (Polish: traktat welawsko-bydgoski).
In exchange for military aid in the Second Northern War and the return of Ermland (Ermeland, Warmia) to Poland, the Polish-Lithuanian monarch granted the Hohenzollern dynasty of Brandenburg hereditary sovereignty in the Duchy of Prussia, pawned Draheim (Drahim) and Elbing (Elbląg) to Brandenburg and handed over Lauenburg and Bütow Land to the Hohenzollerns as a hereditary fief.
