The Treaty of Welawa was a political act signed in the Prussian town of Welawa (German Wehlau) between Poland and Brandenburg-Prussia during the Swedish Deluge on September 9, 1657.

Frederick William I, the "Great Elector", of the Brandenburg Hohenzollerns, promised the Polish king Jan Kazimierz to cancel his alliance with Karl X Gustav of Sweden. In return he was given full sovereignty over Ducal Prussia and the territory ceased to be a Polish fief. In case of an end to the Hohenzollern rule in Prussia, Ducal Prussia was to return to Poland as its integral part.

The treaty was ammended by the Treaty of Bydgoszcz of November 6, 1657.