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The Treaty of Hadiach (Polish: ugoda hadziacka; Ukrainian: гадяцький договір) was a treaty signed on 16 September 1658 in Hadiach between representatives of the Polish–Lithuanian Commonwealth (Stanisław Kazimierz Bieniewski representing Poland and Kazimieras Liudvikas Jevlaševskis representing Lithuania) and Zaporozhian Cossacks (represented by Hetman Ivan Vyhovsky and starshina Yuri Nemyrych, the architect of the treaty, and Pavlo Teteria).
It was designed to elevate the Cossacks and Ruthenians to a position equal to that of Poland and Lithuania in the Polish–Lithuanian union and in fact transforming the Polish–Lithuanian Commonwealth into a Polish–Lithuanian–Ruthenian Commonwealth (Polish: Rzeczpospolita Trojga Narodów, lit. 'Republic of Three Nations', Lithuanian: Trijų Tautų Respublika, Belarusian: Рэч Паспалітая трох народаў, Ukrainian: Річ Посполита Трьох Народів).