In 1569, the Polish–Lithuanian union was transformed into a single state and Słonim became an important regional centre within the newly established Polish–Lithuanian Commonwealth. Administratively it was part of the Nowogródek Voivodeship. Thanks to the efforts of nobleman, statesman and Słonim starost Lew Sapieha, King Sigismund III Vasa renewed the town rights of Słonim and granted the city coat of arms, which included the Lis coat of arms of Sapieha. Also thanks to Lew Sapieha, from 1631 to 1685 the city flourished as the seat of the Lithuanian diet.