Osu Castle (also known as Fort Christiansborg or Christiansborg Castle) is a castle located in Osu, Accra, Ghana, on the coast of the Gulf of Guinea.

A substantial fort was built by Denmark-Norway in the 1660s; thereafter, the fort changed ownership between Denmark-Norway, Portugal, the Akwamu, Britain, and finally post-Independence Ghana. Under Denmark–Norway control it was the capital of the Danish Gold Coast, and held and dispatched enslaved people overseas.

In 1902, Christiansborg Castle (as it then was) became the seat of government in the Gold Coast; however, in modern Ghana, the seat of government has now been moved to Jubilee House.

Because of its testimony to European colonial influence in West Africa and the Atlantic slave trade, the castle was inscribed on the UNESCO World Heritage List in 1979 along with several other castles and forts in Ghana.

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 late 1640s, led by the German trader Heinrich Carloff. 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-Norway. In 1657, Carloff had again traveled to Africa, this time representing Denmark-Norway. 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 Dano-Norwegian control it increasingly dealt with slaves.

Osu Castle was located close to two other forts. Fort Crèvecœur was controlled by the Dutch and Fort James by the British. The settlement at Osu was too small to store sufficient goods to compete with the others. Consequently, Denmark-Norway purchased adjoining land and expanded the building, naming it Fort Christiansborg after the reigning Danish King Christian V. This is not to be confused with the slightly later royal palace in Copenhagen. Denmark-Norway would occupy the fort for most of the next 200 years, with some interruptions, and for much of that time it served as the capital of the Gold Coast of Denmark-Norway.

In 1679 or 1680, the fort's Greek assistant commander incited a mutiny to murder the commander. Shortly after that, a Portuguese ship commanded by Julião de Campos Barreto visited the fort and agreed to purchase it. The fort was named Fort São Francisco Xavier after the Catholic missionary Francis Xavier. The Portuguese built a chapel and raised the bastions by three feet. The fort was abandoned on 29 August 1682 after the garrison mutinied and it became clear that Portuguese traders could not compete with the other Gold Coast powers. Danish forces returned in February 1683 after purchasing the fort back from the Portuguese. In 1685, Fort Christiansborg became the capital of the Gold Coast of Denmark-Norway, taking over from Fort Frederiksborg.

The Akwamu ethnic group occupied the fort in 1693 after overpowering the occupants (who were reduced by death and disease) while disguised as merchants. Assameni, the Akwamu leader, occupied the fort for a year, trading with merchants from many nations. In 1694, Assameni sold the fort back to Denmark-Norway for 50 marks of gold (400 troy ounces, worth £200,000 to £250,000 in 2008) but retained the keys, which are still in the ethnic group's possession to this day. The early 18th century was not kind to the fort, and in 1722 the English reported it to be in disrepair. Extensions were made later that century, however, and structural improvements were made in 1824. The additional store rooms, garrison quarters, platforms, bastions and houses resulted in the castle being four times the size of the original fort. In the 1770s, the Danes at Osu became involved in a conflict with Dutch-controlled Accra.