Sønderborg Castle is located in the town of Sønderborg, Denmark on the island of Als in South Jutland. It houses a museum focusing on the history and culture of the area. The castle is located in the middle of the town, in a park setting overlooking Als Fjord. The museum is open year-round.

Since 1921 Sønderborg Castle has been the home for The Sønderborg Castle Museum, which is the main museum for the former Duchy of Slesvig. The museum houses local and regional history collections from the Middle Ages to the present day, but with especial focus on the Schleswig wars of 1848-50 and 1864, World War II, and the Reunification of 1920.

The museum also hosts exhibitions on navigation, textiles and handicrafts, and has a small art collection with works by prominent Southern Jutland painters over the years.

The original ramparts around the castle became a visible part of the gardens in the 1970s.