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. 
Slagelse is the largest city in the municipality, and is the seat of the municipal council. It is the 21st largest city in Denmark. The city's largest attractions include the sizable shopping center West Zealand Center (Danish: Vestsjællandscenteret), the ruins of Antvorskov and the nearby ring castle of Trelleborg.
Slagelse is located around 15 km east of Korsør and 17 km north-east of Skælskør. 13 km east of Slagelse is Sorø, 33 km north is Kalundborg and 32 km south-east is Næstved. Slagelse borders a large forest area to the south-east, with the closest parts of the forest known as Slagelse Lystskov. Antvorskov barracks and the village of Harrested is located to the south, while the rest of the city borders fields.
Antvorskov and the ruins of the Antvorskov Monastery is located in the southern part of the town, where a school and a large supermarket is also located. The police department of the municipality is also located in the southern part of the town, as is Antvorskov Church. The south-western part of the city consist of residential areas and a number of large shops. The western side of the town is mainly residential with a small industrial area. Also to the west, though closer to the center of the city, is Slagelse Gymnasium and the surrounding educations and schools, including a large vocational school. The hiking trail and former railway Fodsporet cut through the south and western parts of Slagelse. The northern parts of Slagelse have large residential and industrial areas, and is also the location of the Danish Welfarehistorical Museum, several schools and a vocational school. To the east is Skovsø, which is a village that has mostly grown together with Slagelse and is home to many industries. A golf course is also located in Skovsø. Next to Skovsø, at the south-east of Slagelse, are mostly residential areas, and further towards the town center is Slagelse Hospital. The center of Slagelse is the home to a large number of shops, restaurants, cafés and services. Slagelse Library, Slagelse Museum, Slagelse Train Station, the town hall and two churches are located around the center of the city. The West Zealand Center (Danish: Vestsjællandscenteret) is a shopping center originating from 1969 and is also located at the center of the town.
Korsør is located on the south-western tip of Zealand, connecting to Nyborg through the Great Belt Bridge. The city is split in two by the Korsør Nor fjord. The northern half of the city is known as Halsskov, and is home to Korsør Train Station, located in the outskirts of Halsskov, near the village of Stibjerg Huse. Halsskov is home to a small industrial area, a harbour, Halsskov Church, a sizable camping and vacation center and a number of cultural facilities, including a library. The southern half of Korsør is known by the city's name, and is home to a naval base, a harbour, St Povls Church and a small industrial area. The Danish Maritime Authority has its headquarter in Korsør. Also located in the city is Korsør Fortification (Danish: Korsør Fæstning), where a castle from the 1200s used to be located. It was torn down in the start of the 1800s, but the fortifications remain.
Skælskør is located south-east of Korsør and south-west of Slagelse. It borders the fjord of Noret, which goes through the town and split it in two. Skælskør Church is located in the western part of the town. In the eastern part is a sizable industrial area. Denmark Bus Museum is located in Skælskør. The village and beach of Kobæk Strand is located 1 km west of Skælskør.
Forlev-Vemmelev (also known under the individual names Forlev and Vemmelev) are two towns that have grown together. The merged town is referred to under all three names. The Statistics Denmark refer to the town as Forlev while Slagelse Municipality refer to it as Vemmelev. The E20 motorway cut through the town, with the former Forlev located north of the motorway and Vemmelev south of it. A bridge connects the two sides of the town. Vemmelev Church is located in Vemmelev.
The large size and population of the municipality mean that there are many small villages and settlements in the municipality.
Omø Harbour in Kirkehavn.
Slots Bjergby is a village located ca. 2 km south of Slagelse. The village, which has also had the names of Bjergby and Munke Bjergby, is home to the highest located church on Zealand, 77.5 meters above sea level.
Fladholte is a small village located 12 km south-east of Slagelse. The village is located on the border to Næstved Municipality. In the village is the largest private collection of military vehicles in the Nordic Countries, in a museum called PanzerMuseum East. The museum display various types of military vehicles, two helicopters and a Bac 1-11 passenger plane is also displayed in the museum. 
Årslev is located 6 km north of Slagelse. Outside the village, to the east, is a permanent circus and amusement park called Arena Cirkusland, home to Cirkus Arena. 
Omø By is the main settlement on the island of Omø, with the other settlement on the island called Kirkehavn (or Omø Kirkehavn). Omø Church is located in Omø By.
The main settlement on the island of Agersø is called Agersø By. Agersø Church and Agersø Mill are located here.
The island of Glænø is inhabited, though there are no named settlements on the island.
In addition to these villages, there are a number of smaller settlements in the municipality. These are all the settlements with populations of less than 200 people:
Most of the land in the municipality are used for agriculture, leaving little territory for nature. The municipality does have a long coastline with many important spots for especially birds, and there are also a series of forests located around in the municipality.
Tude Å is a river spanning from Trelleborg near Slagelse and out to the coast between Frølunde Fed and Næsby Strand. 208 acres around the river have been protected since 1991. This area is known as Tude Ådal. The river's significance in terms of flora and fauna is limited, but it is a historically important place due to its close ties to Trelleborg and the vikings.
100 acres around the hill of Tårnbjerg were protected in 1993. The hill borders Korsør Nor and is located east of Korsør, west of Tårnby and south-east of Svenstrup and Frølunde.
In the south-west of Slagelse Municipality, bordering Skælskør to the south, is the peninsula of Stignæs. 1,215 acres are protected. The protection was carried out over two stages, first in 1978 and then in 1987. The areas covered in the protection are Stigsnæs, Borreby and Basnæs Nor. The nature of the peninsula include forests, bogs and beach meadows. Doronicum pardalianches is a rare plant in Denmark that can be found on the peninsula. The peninsula is used by many migrating birds, and there are high populations of birds of prey, mainly common buzzard and European honey buzzard. The peninsula is the only breeding spot for the black-tailed godwit on Zealand.
Glænø is located between the fjords of Holsteinborg Nor and Basnæs Nor, with the smaller fjord of Øksenæs Nor located further west. Not all territory in the three fjords are protected, but they are all bird sanctuaries with strict rules in regards to hunting in the area. Parts of Øksenæs Nor is protected under the Stignæs protection, while 276 acres in and around Holsteinborg Nor have been protected since 1936. The Holsteinborg Nor protection also covers the small islands of Ormø and Fuglehøj. 
Basnæs Nor and Øksenæs Nor belong to the same bird sanctuary called Basnæs Nor Sanctuary (Danish: Basnæs Nor Vildtreservat). It is the oldest sanctuary in the country, being established in 1919. Birds that breed in the fjord include common eider, European herring gull, common gull, black-headed gull, Arctic tern and mute swan.
The land of Glænø are mainly cultivated, though also with beach meadows and a sizable forest of 60 acres. Glænø is under the Holsteinborg Nor sanctuary.
Omø is located south of Agersø, 6 km off the coast of Zealand. The island has an area of 4.5 km². South of the village of Kirkehavn is a lake, Omø Lake (Danish: Omø Sø), and further west is a bog simply known as the Bog (Danish: Mosen), with the lake of Mosebækken. The greylag goose and Eurasian bittern breed in the area around Omø Lake.
Sprogø is 154 acres, and is located centrally in the Great Belt. It connects the two halves of the Great Belt Bridge, and the large amount of traffic caused by that limits the amount of wildlife found on the island. Despite that, the island is a popular breeding spot for many species of gulls and terns. The three Danish species of swallows thrive on the island, making home in the buildings and the lighthouse on the island. The sandwich tern, rare in Denmark, live on the island.
The largest industry in the private sector is retail and shops, employing around a quarter of the population hired in the private sector. Other large industries are services such as accounting, counseling, real estate and cleaning. 
Companies with their headquarters in Slagelse Municipality include printing company Nilpeter.Harboe's Brewery is set in Skælskør. Harboe's Brewery is a brewery and factory that produce beer, soda and energi drinks. The company has been situated in Skælskør since it was founded in 1883.
Arla has a dairy factory in Slagelse. Orkla ASA has a factory in Skælskør.
There are 18 ground schools, 2 efterskoler and 8 independent schools in the municipality, as well as 1 youth school and 5 special schools. There are also 1 gymnasium, 1 vocational school and 1 music school.
There are 5 libraries in the municipality, with the main library set in Slagelse. The other libraries are located in Skælskør, Korsør, Agersø and Omø. There are also minor library services in Dalmose and Forlev-Vemmelev.
With several larger towns in the municipality, the sights are spread out all across the municipality. The large number of villages also mean that there are many churches, and a number of manor houses are also found across the municipality.
Arena Cirkusland is a circus, theme park and is the headquarter of Cirkus Arena. The theme park opened in 2006, and is open for the public during the school vacations. It is located by the village of Årslev, north of Slagelse.
Gerlev Play Park (Danish: Gerlev Legepark) is a park and a science center in the field of playing. The park offers a variety of games and events. It is located by the village of Gerlev, south of Slagelse.
Slagelse Park (Danish: Slagelse Lystanlæg) is a park located centrally in Slagelse. It is a green recreational area, with lakes, playgrounds and a maze.
Korsør Fortification (Danish: Korsør Fæstning) is a set of fortifications located centrally in Korsør. It was built in the 1100s. Today there are five buildings on the fortifications, though all are built at different times and only the tower originates from the castle that used to be located on the fortification. The tower is from the 1200s.
The ruins of Antvorskov Monastery.
Guldagergaard is an international ceramic research center located in Skælskør. The center has an associated park.
Kanehøj Post Mill (Danish: Kanehøj Mølle) is a post mill located outside Skælskør. It originates from 1881.
Agersø Mill (Danish: Agersø Mølle) is a mill from 1892 located on Agersø.
Korsør Mini Town (Danish: Korsør Miniby) is a miniature model of the town of Korsør as it looked in 1875. It is built in 1:10 scale and has existed since 2001.
The Antvorskov Monastery Ruins are located in southern Slagelse. They are the ruins of Antvorskov Monastery, a monastery built in 1164 by Valdemar I. It was the first Knights Hospitaller monastery in Denmark, and was used as monastery until 1536 when the crown took over ownership and turned into a castle. Frederik II used the castle between 1580-1584. After that it was used by fief lords until 1717, when it became a ryttergods - a location for the Danish cavalry. It was sold in 1774 and most of the castle was torn down in 1816.
Skovsgaard's Mill- and Bakery Museum (Danish: Skovsgaard Mølle- og Bagerimuseum) is a bakery museum located in the village of Fladholte, 12 km south-east of Slagelse. The museum covers the period between 1900–2000 and display a variety of bakery-related items. The museum also show a reconstruction of a bakery from 1880. The museum also include a modern bakery, where they sell bread.
Korsør City and Strait Crossing Museum (Danish: Korsør By og Overfartsmuseum) is a museum located in Korsør. The museum was founded in 1983, and is focussed on the strait crossing from Korsør to Nyborg. The museum is located in a building on Korsør Fortification.
Ice Boat Museum (Danish: Isbådsmuseet) is a museum about the iceboats used as transport across the Great Belt during winter. It is located west of Korsør, by the Great Belt Bridge.
Slagelse Museum is located centrally in Slagelse. The museum focuses mainly on trade and artisanry. It includes a reconstruction of a shop from the 1940-1950s, as well as sections on local history. A part of the museum is dedicated to H. C. Andersen, who went to school in Slagelse.
Flakkebjerg School Museum (Danish: Flakkebjerg Skolemuseum) is a museum located in Flakkebjerg. The museum focuses on schools in the period 1904–1962, and has existed since 1981.
Skælskør Town Museum (Danish: Skælskør Bymuseum) is a local museum located in Skælskør. It focuses mostly on trade, maritime transport and fishery.
Denmark's Bus Museum (Danish: Danmarks Busmuseum) is a museum about busses in Denmark. It is located in Skælskør.
Danish Welfarehistorical Museum (Danish: Dansk Forsorgshistorisk Museum) is a museum in Slagelse about the history of welfare in terms of treatment of the mentally ill. 
Havrebjerg Museum is located in Havrebjerg. It focuses on the local history of the village.
Trelleborg is a museum and ring castle located west of Slagelse, between the villages of Hejninge, Gammel Forlev and Næsby ved Stranden. The ring castle was built around year 980 by Harald Bluetooth. The museum exhibits items and discoveries from the ring castle, and educates about the vikings and the Viking Age. The remnants of the ring castle are associated with the museum, and viking battle reenactment occasionally take place around the ring castle. A yearly viking-themed market takes place in Trelleborg's grounds, and also includes a large viking battle reenactment. 
Slagelse Municipality and the Danish National Museum are funding a project to rebuild part of the ring castle. The project is expected to be finished in 2022. 
PanzerMuseum East is a museum for military vehicles located 12 km south-east of Slagelse in the small village of Fladholte. The museum holds the largest private collection of military vehicles in the Nordic Countries. The owner of the museum is Allan Pedersen, who opened the museum in 2014. Most of the vehicles and exhibited items in the museum have been restored by Pedersen, who opened the museum after having privately owned and restored the collection for years. The museum span 4,500 km², and include a large park area with 3 shelter groups. In the park are also nordmann fir available during christmas for christmas trees. The museum hosts a yearly market known as History and Lifestyle (Danish: Historie og Livsstil), which takes place in the park surrounding the museum. 
The museum focuses primarily on military vehicles, though also display military uniforms and a variety of military memorabilia. Vehicles in the museum include T-72, BMP-1, OT-90, UAZ-469, ZIL-130, LuAZ-967 and more. 
There are three aircraft in the museum, two helicopters and a passenger plane. The helicopters are a Mi-24P Hind helicopter and a Hip Mi-8 helicopter. The passenger plane is a Bac 1-11.
The important central location of the municipality, in terms of agriculture and crossing of the Great Belt, has made it an ideal location for manor houses. Today there are still many manors scattered around the municipality.
Basnæs is a manor located south of Skælskør. The manor used to be two villages, named Basnes and Nybølle. These were merged in the 1400s to form the Basnæs manor. The manor has switched owners frequently throughout its history.
Bonderup is a manor located south-east of Korsør. It was built in the middle of the 1700s. Bonderup has been owned by the Moltke family since 1825.
Borreby is a manor and castle located south of Skælskør. Like many manor houses in the area it used to be a village. The manor is known from at least 1344. It has since 1783 been owned by the Castenschiold family.
Brorupgaard is a manor located south of Havrebjerg and north of Slagelse. The manor is first mentioned in 1454, but is known to have existed since the Middle Ages. Between 1740-1754 it was owned by Ludvig Holberg. It has since 1855 been owned by the Bech family.
Charlottendal is a manor located south of Slagelse. It was a part of the Antvorskov Monastery, and later of the Antvorskov Cavalry District (Danish: Antvorskov Rytterdistrikt) until 1799 when Antvorskov was split up into several manors, one of them being Charlottendal. The manor is today part of Antvorskov barracks.
Espe is a manor located south of Korsør and south-west of Boeslunde. The manor is known from at least 1561, and was owned by the crown before that. The manor has changed owners many times throughout history, but has since 1810 been owned by the Moltke family.
Falkensteen is a manor located south of Slagelse. It is known to have existed since at least 1372 under the name of Pebringegaard. It was renamed to Falkensteen around the end of the 1700s, named after the manor's owner's wife, Anne Mathea Falck.
Gerdrup is a manor located in the village of the same name. The earliest mentioning of Gerdrup comes from 1417, though it is likely much older than that. It has been owned by the Fabricius family since 1919. 
Gyldenholm is a manor located south-east of Slagelse. It is a young manor, established in 1774.
Idagaard is a manor located in Slagelse. It was a part of Antvorskov Cavalry District (Danish: Antvorskov Rytterdistrikt) until 1799 when Antvorskov was split up into several manors, one of them being Idagaard.
Lyngbygaard is a manor located between Boeslunde and Eggeslevmagle. Lyngbygaard was originally a village named Lyngby, though it was burned down during the Dano-Swedish War in 1657-1658. The manor was established in 1700, and has been a part of the Gerdrup manor through most of its history, and still is today.
Snedinge is a manor located north of Glænø, between Skælskør and Bisserup. It has existed since 1370 and has been a part of the Holsteinborg manor since 1708.
Store Frederikslund is a manor located north-east of Slagelse and north-west of Sorø. It was established in 1783. In 2012 the manor was sold for 200 million DKK, which made it one of the largest real estate trades in Denmark's history.
Taarnborg is a manor located in Halsskov in Korsør. Taarnborg was originally a village, in which a castle was built in the 1100s. It was referred to as Korsør Castle and later came under the name of Dyrehovedgaard. It was renamed to Taarnborg in 1846. The manor owns 334 acres.
Taarnholm is a manor located east of Korsør, across Korsør Nor. It is located between the villages of Hulby, Ormeslev and Halseby. It was established in 1774, before that being a part of the Antvorskov Cavalry District (Danish: Antvorskov Rytterdistrikt). With the district dissolved, the territory was auctioned off, and was bought by Christian Eggers who established the Taarnholm manor. The two churches of Hemmeshøj and Taarnborg were also part of the manor, though barely any of the land was cultivated or built up, having been used for grazing horses. Christian Eggers' son inherited the manor, and in 1796 he sold Taarnholm to the owner of the Nordruplund manor. It was sold again in 1805, this time to a consortium. The consortium dissolved in 1819, where Vilhelm Carl Ferdinand Ahlefeldt-Laurvigen became the sole owner of the manor. It was sold off to the National Bank in 1835, but was sold off again already in 1836. The manor is today used for agriculture and own 465 acres.
Valbygaard is a manor located north-west of Slagelse. It is known to have existed since the Middle Ages, and was until 1774 owned by the crown. It has been in the Bech family since 1846.
Holsteinborg is a manor and castle located south of Rude, north-west of Bisserup and north-east of Glænø. Holsteinborg has existed since at least the early 1300s, though a castle has likely been located at Holsteinborg's location since the 1100s, where it would have been a defensive castle. In the 1300s Holsteinborg was named Braade. It was owned by the church until 1536 where it was taken by the crown. In 1562 it was sold off to Niels Trolle, brother of Herluf Trolle. Niels Trolle died in 1565 in the Seven Years' War, and Braade was inherited by his widow and children. One of his children, Børge Trolle, bought out the other heirs and renamed the manor to Trolholm. It remained under the Trolle family until 1707, where it was sold to Ulrich Adolph Holstein, who renamed the manor to Holsteinborg. He also owned the manors of Snedinge and Fyrendal. Both of these came under Holsteinborg in 1708. It has since then remained under ownership of the Holstein-Holsteinborg family.
The total area of Holsteinborg's land amount to 1,386 acres. Of these are 800 used for agriculture and 546 for forests.
Slagelse Celebration Week (Danish: Slagelse Festuge) is a yearly week-long event, which takes place in Slagelse. Through the week there are live music and entertainment, as well as a running race through the city.
Sørby Market (Danish: Sørby Marked) is a yearly kræmmermarked - a type of flea market. It takes place in Sørbymagle. Along with the various booths through the market, there are also live music, food and entertainment.
Skælskør Ceramics Festival (Danish: Skælskør Keramikfestival) is a yearly arrangement focused around ceramics. The arrangement includes a market and many displays of ceramics by various artists.
A yearly viking-themed market takes place on Trelleborg's grounds, and also includes a large viking battle reenactment, as well as various types of entertainment and education.
PanzerMuseum East hosts a yearly market known as History and Lifestyle (Danish: Historie og Livsstil), which takes place in the park surrounding the museum. It's a market
with focus on lifestyle, with history-focussed entertainment and education.
There are 41 parishes in Slagelse Municipality. Shown in the table below are the populations of each parish, as well as the percentage of that population that are members of the Church of Denmark. All numbers are from 1 January 2020.
The coat of arms of Slagelse Municipality shows the Great Belt Bridge in green on white background, with blue wavy lines under the bridge to represent the Great Belt. The bridge represents cooperation and balance with the surrounding nature. It also shows the municipality as a forward-looking municipality with high ambitions, with people and technological progress in focus. It is also meant to be a clearly distinguishable feature. It was adopted in 2009.
The coat of arms doesn't resemble any previous coat of arms from the area and is a unique design from 2009. Below are all coats of arms of the cities of the municipality.