Osu Castle, also known as Fort Christiansborg or simply the Castle, is a castle located in Osu, Accra, on the coast of the Atlantic Ocean's Gulf of Guinea. The first substantial fort was built by the Danish in the 1660s, though the castle has changed hands between Denmark, Portugal, the Akwamu, the British, and finally post-Independence Ghana, and was rebuilt numerous times. For most of the castle's history it has been the seat of government in Ghana with some interruptions, the latest when the John Kufuor administration moved the seat of government to Golden Jubilee House after 6 January 2009, which was quickly reversed by the incoming John Atta Mills administration.

The area was first occupied in 1550 by the Portuguese, though in the 17th century Portuguese influence diminished. The area came under the control of Sweden in the 1650s, led by the Dutch trader Henry Caerlof. In 1652 he was given permission to build a small fortified lodge by the King of Accra, with whom he had previously done business. In 1660 control passed to the Netherlands but it was soon lost to Denmark. In 1657 Caerlof had again traveled to Africa, this time representing Denmark. He aimed to conquer the forts he had previously established, which he found easy at Osu. In its early life the castle was primarily used in the gold and ivory trade, but under Danish control it increasingly dealt with slaves.