Akershus Fortress (Norwegian: Akershus Festning) or Akershus Castle (Norwegian: Akershus slott) is a medieval castle that was built to protect Oslo, the capital of Norway. It has also been used as a prison.

It is not known exactly when the construction of the castle started but it is believed that it happened around the late 1290s, by King Haakon V, replacing Tønsberg as one of the two most important Norwegian castles of the period (the other being Båhus). It was constructed in response to the Norwegian nobleman, Earl Alv Erlingsson of Sarpsborg’s earlier attack on Oslo that occurred in 1287. In the aftermath of the attack, it became clear that the city’s existing defences weren’t effective and therefore, a stronger defensive centre was needed.

The castle is mentioned in written sources for the first time in 1300 in a letter from king Haakon to a church in Oslo. However, the letter does not mention how far the construction of the castle has progressed by then.