Uusikaupunki (Nystad in Swedish) is a town and municipality of Finland.

It is located in the province of Western Finland and is part of the Finland Proper region. The municipality has a population of 16,405 (2003) and covers an area of 540.65 km² of which 47.72 km² is water. The population density is 33.3 inhabitants per km².

The municipality is unilingually Finnish.

Uusikaupunki is the home of Valmet Automotive, a mechanical production company, producing cars and vehicles for other manufacturers.

In 1721, the Peace of Nystad was signed in Uusikaupunki.