Slagelse Municipality is a kommune in Region Zealand on the west coast of the island of Zealand in Denmark. The municipality covers an area of 571 km2. The municipality borders Kalundborg Municipality to the north, Sorø Municipality to the north-east, Næstved Municipality to the south-east and connects to Nyborg Municipality via the Great Belt Bridge.

Zealand at 7,031 km2 is the largest and most populous island in Denmark proper. Zealand had a population of 2,319,705 on 1 January 2020, comprising 40% of the country's population.