Slagelse is a town on Zealand, Denmark. The town is the seat of Slagelse Municipality, and is the biggest town of the municipality. It is located 15 km east of Korsør, 16 km north-east of Skælskør, 33 km south-east of Kalundborg and 14 km west of Sorø.

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.