The Asahan River (Indonesian: Sungai Asahan) is one of the principal rivers in North Sumatra, Indonesia, that begins in Porsea, Toba Regency near the southeast corner of Lake Toba.

[2] The river flows in a north-easterly direction, cutting through the upper course of the deep valley in the Barisan Mountains in Toba Regency, then flows through the Asahan Regency and eventually empties into the Strait of Malacca The largest city on the Asahan is Tanjungbalai, Asahan with a population of more than 180,000 people.

The river houses the Sigura-gura Dam, which supplies power to the North Sumatran Province.

Tributaries of the Asahan include the Silang, Silau, Nantalu, Masihi, Lauran, and Baru River.

The warmest month is March, when the average temperature is around 25 °C, and the coldest is May, at 23 °C.