Olsborg Castle (Swedish: Olsborg), also Olofsborg, was as a fortified castle located in Tanum Municipality, Sweden, in the province of Bohuslän. It is located on a steep cliff, and might previous to later use been an early hill fort. It was constructed in 1503 or 1504 by the squire Nils Ragvaldsson from Åby, after a recent Swedish attack on Viken.