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.

Riau is a province of Indonesia. It is located on the central eastern coast of Sumatra along the Strait of Malacca. The province shares land borders with North Sumatra to the northwest, West Sumatra to the west, and Jambi to the south, and a maritime border with the Riau Islands and the country of Malaysia to the east. It is the second-largest province in the island of Sumatra after South Sumatra, and is slightly larger than Jordan. According to the 2020 census, Riau had a population of 6,394,087 across a land area of 89,935.90 km²; the official estimate of population as of mid-2022 was 6,614,384. The province comprises ten regencies and two cities, with Pekanbaru serving as the capital and largest city.