Uusikaupunki (ōˈsēkouˌpŏngkē) [key], Swed. Nystad, city (1996 pop. 17,631), Western Finland prov., SW Finland, on the Gulf of Bothnia. A local trade center and port, it has sawmills, machine shops, granite quarries, and a shipyard. It was chartered in 1616 and was the scene (1721) of the signing of the Treaty of Nystad (see Northern War).