The Sultanate of Siak Sri Indrapura, often called Sultanate of Siak, was a kingdom that was located in present-day Siak Regency, and nearby other regions from 1722 to 1949. It was founded by Raja Kecil, who had close relations with the Johor Sultanate, after he failed to seize the Johor throne. The polity expanded in the 18th century to encompass much of eastern Sumatra as it brought various communities under its control through warfare and control of trade between the interior of Sumatra and the Malacca Strait. The Dutch colonial state signed a series of treaties with the Siak rulers in the 19th century, which reduced the area of state influence to the Siak River. For the remainder of the Dutch colonial era, it operated as an independent state with Dutch advisors. After Indonesia's Independence was proclaimed on 17 August 1945, the last sultan of Siak, Sultan Syarif Kasim II, declared his kingdom to join the Republic of Indonesia.

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.