Siak, is a regency (kabupaten) of Riau Province, on the island of Sumatra, Indonesia. It has an area of 8,556.09 km2 and had a population of 376,742 at the 2010 Census and 457,940 at the 2020 Census; the official estimate as at mid 2022 was 477,550, comprising 246,186 males and 231,364 females. The administrative centre of the regency is located at Siak Sri Indrapura. The northern part of this regency contains a large part of the Bukit Batu Biosphere Reserve.