Kisimul (also Kiessimul or Chisimul) Castle is a small medieval castle located in the centre of Castlebay on Barra, an island of the Outer Hebrides, Scotland.
Construction of Kisimul Castle was started by Niall of Barra around 1039. Since that time, Kisimul has been the stronghold of Clan MacNeil. It is completely surrounded by water and has never fallen to an enemy. The name of the castle comes from the Gaelic words cios (tax) and mul (mound).
Kisimul was abandoned in 1838 when the island was sold, and the castle's condition deteriorated. Some of its stone was used as ballast for fishing vessels, and some even ended up as paving in Glasgow. The remains of the castle, along with most of the island of Barra, were purchased by the chief of Clan MacNeill in 1937, who made efforts at restoration.
In 2001 the castle was leased by the chief of Clan MacNeil to Historic Scotland for 1000 years for the annual sum of £1 and a bottle of whisky.