Nordborg Castle is a small castle which lies on the southern side of Nordborg Lake in Nordborg on the island of Als in Sønderborg Municipality, Denmark. The castle premises are currently used by Nordborg Slots Efterskole as a boarding school. Nordborg Castle has also been the backdrop for a number of years for Nord-Als Musikfestival.

Nordborg, is a town with a population of 5,759, which was the seat of the former Nordborg municipality in Sønderborg Municipality, Region of Southern Denmark on the northwest half of the island of Als off the east coast of the Jutland peninsula in south Denmark.