Sønderborg (German: Sonderburg), is a municipality (Danish, kommune) in Region Syddanmark partially on the Jutland peninsula and partially on the island of Als in south Denmark. The municipality covers an area of 499 km² and a total population of 76,825 (2007). Its mayor is Jan Prokopek Jensen, a member of the Socialdemokraterne political party.
The site of its municipal council is the town of Sønderborg.
The municipality is split into two sections separated by Als Fjord, the waterway which separates the island of Als from the Jutland mainland.
The town of Sønderborg lies on both sides of Als Fjord; the narrow strait between these two sides is called Als Strait (Alssund). Two bridges connect the city across the strait: the 682 meter long Als Strait Bridge (Alssundbro), built in 1978-1981, carries automobile traffic; and the 331 meter long King Christian X's Bridge (Kong Christian Xs Bro), built in 1925-1930, carries also automobile traffic