Alas River is a river in Aceh in northeastern Sumatra, Indonesia, about 1400 km northwest of the capital Jakarta.

[3] The banks of the rivers are inhabited by the Alas people.

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

The warmest month is March, when the average temperature is around 23 °C, and the coldest is June, at 21 °C.

The wettest month is December, with an average of 370 mm rainfall, and the driest is June, with 82 mm rainfall.