Altenhus Fortress (Norwegian: Altenhus festning; also Kongshus Fortress) was a fortification built on the island of Årøya in the Alta area of Finnmark, Norway in 1610. Altenhus Fortress was intended to prevent Swedish incursions into the area, and secure the rich salmon fisheries for the Dano-Norwegian state. The structure only remained in military use for three years, before being abandoned and used as a prison until 1692, when the timber of the fortress' blockhouse was used in the construction of a church.