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.

The fortress has successfully survived all sieges, primarily by Swedish forces, including those by forces led by Charles XII in 1716. In the early 17th century, the fortress was modernized and remodelled under the reign of the active King Christian IV, and got the appearance of a renaissance castle.

The fortress was first used in battle in 1308, when it was besieged by the Swedish duke Eric of Södermanland, whose brother won the Swedish throne in 1309. The siege was eventually broken by a local Norwegian army in a battle. (This battle forms a forms a major part of the plot of Sigrid Undset historical novel "In the Wilderness", the third volume of her tetralogy The Master of Hestviken.)

In 1449-1450 the castle was besieged again, this time by the Swedish king Karl Knutsson Bonde, but he had to lift the siege after a while. The castle wasn’t besieged again until 1502 when Scottish soldiers in service of the Danish king besieged the castle in order to regain it from the hands of the Norwegian nobleman Knut Alysson.

Akershus was besieged yet again in 1523, this time by Swedish soldiers but Oslo’s inhabitants burned down their houses in an attempt to drive them out and the Swedes retreated after a short period.

The king Christian II besieged the castle from 1531 to 1532 but the siege was lifted by forces from Danmark and Lübeck. After this siege the castle was improved and strengthened.

During the Northern Seven Years' War, the castle was besieged again in 1567 by Swedish forces, but Christen Munk burned down the city in a manner that deprived the Swedes from receiving supplies and strengthening themselves and eventually they retreated.

The immediate proximity of the sea was a key feature, for naval power was a vital military force as the majority of Norwegian commerce in that period was by sea. The fortress was strategically important for the capital, and therefore, Norway as well. Whoever controlled Akershus fortress ruled Norway.

The fortress has never successfully been besieged by a foreign enemy. However it surrendered without combat to Nazi Germany in 1940 when the Norwegian government evacuated the capital in the face of the unprovoked German assault on Denmark and Norway (see Operation Weserübung). During World War II, several people were executed here by the German occupiers, including members of the Pelle group. The fortress was liberated on 11 May 1945, when it was handed over to Terje Rollem on behalf of the Norwegian resistance movement. After the war, eight Norwegian traitors who had been tried for war crimes and sentenced to death were also executed at the fortress. Among those executed were Vidkun Quisling and Siegfried Fehmer.

After the main building has undergone restoration, it has been used for official events and dinners for dignitaries and foreign heads of state.