The Siak (Sungai Siak) is a river of Riau province, in the east of Sumatra, Indonesia, about 1000 km northwest of the capital Jakarta.

[2] Approximately 370 kilometres in length, the Siak is a blackwater river which owes its brown color to dissolved organic matter (DOM) leached from surrounding, heavily disturbed peat soils; it is heavily polluted, notably by the petroleum industry.

It flows through the town of Pekanbaru before emptying into the Strait of Malacca.

[3] The river flows in the central area of Sumatra with predominantly tropical rainforest climate (designated as Af in the Köppen-Geiger climate classification).

The warmest month is October, when the average temperature is around 25 °C, and the coldest is January, at 22 °C.