Sønderborg is a Danish town in the Region of Southern Denmark. It is the main town and the administrative seat of Sønderborg Municipality (Kommune). The town has a population of 28,137, in a municipality of 74,380. In recent times, Sønderborg is a center for trade, tourism, industry, and education in the region of Southern Denmark. The town is the headquarters for several industrial companies. Sønderburg joined the UNESCO Global Network of Learning Cities in 2016.

Sønderborg Municipality, is a kommune in the Region of Southern Denmark partially on the Jutland peninsula and partially on the island of Als in south Denmark, at the border with Germany. The municipality covers an area of 495.86 km2 (191.45 sq mi), and has a population of 74,380. Its mayor as of 1 January 2014 is Erik Lauritzen, a member of the Social Democratic party.