Siak is a regency (kabupaten) of Riau Province, on the island of Sumatra, Indonesia. It has an area of 8,216.06 km² and population of 377,232 at the 2010 Census. The northern part of this regency is covered by the Bukit Batu Biosphere Reserve.Previously the area was part of the Sultanate of Siak Sri Indrapura. At the beginning of the independence of Indonesia, Sultan Syarif Kasim II, the Sultan of Siak last states to join the kingdom of the Republic of Indonesia.