Mäntsälä is a municipality in the province of Southern Finland, and is part of the Uusimaa region. It has a population of 20,933 and covers an area of 596.11 square kilometres of which 15.26 km² is water. The population density is 36.04 inhabitants per square kilometre. The municipality is unilingually Finnish. Mäntsälä is one of three municipalities in the Uusimaa region that do not have a Swedish name; the others are Nurmijärvi and Askola.