Slagelse Municipality (Danish: Slagelse Kommune, pronounced [ˈslɛːjl̩sə kʰoˈmuːnə]) 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.

The main city and the site of the municipal council is the city of Slagelse. The mayor was earlier John Dyrby Paulsen, a member of the Social Democratic political party.

The municipality include the inhabited islands of Omø, Agersø and Glænø, as well as the uninhabited islands of Sprogø, Østerfed, Stenfed, Ormø, Fuglehøj, Sandholm, Kidholm and Fugleholm.

On 1 January 2007 the former Slagelse municipality was, as the result of Kommunalreformen ("The Municipal Reform" of 2007), merged with Hashøj, Korsør, and Skælskør municipalities to form an enlarged Slagelse municipality.

Slagelse has existed since the 1000s, and coins have been minted there since Cnut the Great's time. It was granted the status of market town by Eric V in 1288. Skælskør was given market town status in 1414. Korsør was granted the same status in 1425. While Slagelse and Skælskør were already important locations before becoming a market town, with the ring castle of Trelleborg located next to Slagelse, Korsør was a more insignificant town before becoming a market town. Korsør's location quickly made it an important town for ferry services to Nyborg on Funen. The three towns became competitors for the land and resources. Korsør's importance as a way to get to Funen allowed Korsør to be favoritized over Slagelse until the 1600s. The town remained small, but fared better than Slagelse, who was struggling. This was also linked to Korsør's access to the sea, giving them a variety of opportunities and privileges, including being able to ship out the grain from many farm areas on western Zealand, leaving them in conflict with the neighboring towns. Slagelse's economy was meanwhile mainly based on artisanry and tobacco. Skælskør was a lesser part of the conflict on western Zealand.

Through the Middle Ages the three towns remained market towns, with the remaining territory of the current municipality going under the administrative hundreds of Slagelse Hundred and Vester Flakkebjerg Hundred. These belonged to the syssel of Vestersyssel. In 1660 the syssel was changed and split up into two counties (Danish: amter). These were Antvorskov County and Korsør County, and they approximately made up the current borders of Slagelse Municipality. In 1798 these counties were merged with Sorø County. In the 1970 Danish Municipal Reform the administrative division of the region was changed again. Market towns were dissolved, and Slagelse instead became the seat of the new Slagelse Municipality. Korsør became the seat of Korsør Municipality, Skælskør of Skælskør Municipality and Dalmose of Hashøj Municipality. All four municipalities were in the new West Zealand County. In the Municipal Reform of 2007 these four municipalities were merged to form the current municipality, which became a municipality in the new Region Zealand.

The table below shows the historical municipal subdivisions of Slagelse Municipality.

Below are all settlements in the municipality with populations of at least 200 people (populations as of 2020).