The Trelleborg (or Trælleborg) west of Slagelse on the Danish island of Zealand, is one of seven Viking ring castles discovered as of 2014. In its day, the fortress was situated on a peninsula that jutted into the swampy area between two rivers. The swamp was connected to the Great Belt by a lake that at its time could be navigated by Viking ships. Trelleborg is believed to have been ordered by King Harald Bluetooth in the year 980 AD and it might have commanded the Great Belt and its sea traffic, between the islands of Zealand and Funen.

Trelleborg is the best preserved of the Viking ring fortresses and there is a museum here since 1995 - Trelleborg Museum -, presenting the story of this particular fortress and the nearby area. Some of the artifacts found in connection with the archaeological excavations are on display at the museum, while others are on display at the Nationalmuseet in Copenhagen. Slagelse Municipality has recently granted DK 25 mio. to update Trelleborg Museum with digital and virtual technology, in a project known as Ny Trelleborg. In collaboration with the other Viking ring castles, the project is also applying for admission to UNESCO as a World Heritage Site.