Sønderborg (German: Sonderburg, Sønderjysk: Synneborre / Synnebo) is a town and municipality in Southern Jutland with 27,841 inhabitants (2020) on both sides of Alssund. Most of the town is located on the island of Als, which is connected to Southern Jutland by King Christian X's Bridge and the northern Alssund Bridge. It is Sønderjylland's largest city and grew up around Sønderborg Castle, which was built a little before the year 1200. Archaeological excavations in various places in the city show the medieval Sønderborg on Humletorvet.

Shipping was an important profession from the 16th century, when ships from Sønderborg sailed throughout the Baltic Sea area with goods from Als. The Prussian bombing in 1864 hit Sønderborg very hard, where more than 80 percent of the town's houses burned down or were damaged. The reconstruction took place slowly, and only after 1900 did the population grow noticeably, e.g. due to German immigration.

Southern Jutland belonged to Germany in the period 1864-1920, and the city lost many citizens who emigrated to Denmark, which lay north of the Kongeå border. In return, Sønderborg gained several German military institutions, and the city's industry and trade developed further. After the Reunification in 1920, a number of Germans emigrated, and the population dropped again from 11,500 before World War I to 8,500 in 1921. The port and road network were expanded, and Christian X’s Bridge replaced the old pontoon bridge. The city grew especially after World War II, and from 1960 it had overtaken Haderslev in terms of population and had become South Jutland's largest city. Half of the city's inhabitants were employed in handicrafts and industry in 1960, and the metal and machine industry in particular were significant industries.


In 1970, with the municipal reform, Sønderborg lost its status as a county seat, and the population fell until the mid-1980s, after which the trend reversed.

The Sønderborg area today has a dynamic business environment within high technology, machinery, food and textile industry and has many educational institutions and colleges. The University of Southern Denmark in Sønderborg offers i.a. business economics, business languages ​​and engineering educations, and in addition there are e.g. Sønderborg Idrætshøjskole og kunstskole. The city is also home to the South Jutland Symphony Orchestra. Sønderborg Hospital is the city's largest workplace with 1,500 employees.

Sønderborg is the capital of Sønderborg Municipality, which is part of the Region of Southern Denmark. On 1 January 2007, Sønderborg became part of the larger Sønderborg Municipality. The municipality currently has 74,561 inhabitants (2019). Other cities in Sønderborg Municipality are: Nordborg, Augustenborg, Guderup, Fynshav, Høruphav, Vester Sottrup, Broager, Havnbjerg and Gråsten.