Slotsholmen (English: The Castle Islet) is an island in the harbour of Copenhagen, Denmark, and part of Copenhagen Inner City. The name is taken from the successive castles and palaces located on the island since Bishop Absalon constructed the city's first castle on the island in 1167 at the site where Christiansborg Palace lies today.

Recognised as the centre of the Government of Denmark since the Middle Ages, the island is sometimes referred to as 'the Island of Power' (Magtens Ø), and is lined with central government institutions and ministries; the name Slotsholmen is thus also frequently used as a metonym for overall Danish governmental administration.

The island is dominated by the vast Christiansborg Palace which houses the Danish Parliament, the Supreme Court of Denmark, the Prime Minister's Office and the State Rooms of the King. Also located on the island are the most important ministries, the Danish National Archives, the Royal Danish Library, several museums and the historic buildings of the Christian IV's Stock Exchange, the Chancellery and Christian IV's Brewhouse.