Akershus Fortress (Norwegian: Akershus Festning) is the old castle built to protect Oslo, the capital of Norway.

The first work on the fortress started around 1290, after the Norwegian nobleman Alv Erlingsson had attacked Oslo. The fortress has survived many sieges, primarily Swedish, and, with the exception of WWII, never been conquered by a foreign enemy.

The fortress was first used in battle in 1306. The immediate proximity of the sea was imperal, for the navy was a vital military force. The fortress was strategically important for the capital, and therefore, Norway as well. Whoever was ruler at Akershus fortress, was ruler of Norway.

The Norwegian Armed Forces museum, the Norwegian Resistance museum, and a military drafting office is present at Akershus fortress.