Sønderborg County is a former province in Denmark, located on the island of Als and the easternmost part of Southern Jutland. Sønderborg County was established in 1920 following the reunification of Denmark and South Jutland following the Schleswig Plebiscites. It was dissolved in 1970 when it merged with three other counties to form South Jutland County. Sønderborg County corresponded geographically to the former Prussian Kreis Sonderburg.
After Denmark surrendered control over Schleswig to Prussia and the Austrian Empire in 1864, the Schleswig island of Ærø was transferred from Augustenborg County to Svendborg County and the island remained Danish. The rest of Augustenborg County was merged with Sønderborg and became Kreis Sonderburg. Sønderborg County and Aabenraa County merged in 1932, forming Aabenraa-Sønderborg County.