The Treaty of Bromberg or Treaty of Bydgoszcz was a treaty between John II Casimir of Poland 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. Thus, the Treaty of Bromberg is sometimes referred to as treaty of Wehlau-Bromberg or Treaty of Wehlau and Bromberg.

Frederick William was Elector of Brandenburg and Duke of Prussia, thus ruler of Brandenburg-Prussia, from 1640 until his death in 1688. A member of the House of Hohenzollern, he is popularly known as "the Great Elector" because of his military and political achievements. Frederick William was a staunch pillar of the Calvinist faith, associated with the rising commercial class. He saw the importance of trade and promoted it vigorously. His shrewd domestic reforms gave Prussia a strong position in the post-Westphalian political order of Northern-Central Europe, setting Prussia up for elevation from duchy to kingdom, achieved under his son and successor.