Mäntsälä is a municipality in the province of Southern Finland, and is part of the Uusimaa region. It has a population of 20,957 (31 December 2023) and covers an area of 596.11 square kilometres (230.16 sq mi) of which 15.26 km2 (5.89 sq mi) is water. The population density is 36.08 inhabitants per square kilometre (93.4/sq mi). 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.

Uusimaa is a region of Finland. It borders the regions of Southwest Finland, Tavastia Proper (Kanta-Häme), Päijänne Tavastia (Päijät-Häme), and Kymenlaakso. Finland's capital and largest city, Helsinki, along with the surrounding metropolitan area, are both contained in the region, and Uusimaa is Finland's most populous region. The population of Uusimaa is 1,734,000.