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.

Bengkalis Regency is a regency of Indonesia in Riau Province. The regency was originally established in 1956 and then included most of the northern part of the province, but on 4 October 1999 it was divided up, with most of the territory being split off to form the new Rokan Hilir Regency, Siak Regency and the city of Dumai. On 19 December 2008 a further five districts were removed to create the new Meranti Islands Regency, leaving eight districts in the Bengkalis Regency. These eight districts has increased since 2010 to eleven by the splitting of existing districts on the Sumatran mainland.