– died 1575 or 1576), coat of arms Topór, was a Polish statesman, kasztelan of Sandomierz and Wojnicz, a delegate to the Sejm of 1569 and one of the signatories of the Union of Lublin, which formally united Poland and Lithuania into the Polish–Lithuanian Commonwealth.

During this time, most likely in Germany, he became familiar with Calvinism, converted, and would remain a faithful adherent and advocate for Protestantism throughout his life.

As an "Executionist", Ossoliński promoted the return of state lands illegally seized by some magnates back to the crown, an increase in the powers of the lesser and medium nobility (szlachta), a curtailment of the rights of the Catholic clergy, enforcement of separation of powers in regard to the holding of public offices, religious tolerance, and reform of public finances.

[2][5] Before the treaty was implemented, Ossoliński argued that Poland and Lithuania were already technically united through the agreements made between Polish king Władysław II Jagiełło (originally from Lithuania) and the Grand Duke Vytautas, and was probably the author of a political pamphlet De unione Lithwaniea ("On the Lithuanian Union").

[4][6] Along with Rafał Leszczyński, the voivode of Brześć Kujawski, Ossoliński proposed the establishment of a national church in Poland, with king Zygmunt August as its head.

[1] He died around 1576 and was buried in Goźlice, in a church which he had converted into a Calvinist Zbór (in Poland–Lithuania and Bohemia a type of Protestant place of worship).