Nordborg Castle (Danish: Nordborg Slot) is a small castle which lies on the southern side of Nordborg Lake in Nordborg on the island of Als in Denmark. The castle premises are currently used as a boarding school.
Nordborg Castle was, according to Saxo, founded by Svend Grathe under the name Alsborg. Hence, this can be dated to around 1150.
Alsborg was built whilst the Wends still dominated the Danish coast; its location a few kilometers inland meant that the castle could not be attacked without warning, and the local population had a better chance of taking refuge there.