The Ashchysu (Kazakh: Ащысу) is a river in the Abai Region, Kazakhstan and a tributary of the Shagan, of the Irtysh basin.

[1] The Ashchysu has its sources in the northeastern slopes of the Chingiztau, a range of the eastern sector of the Kazakh Uplands, about 30 km (19 mi) northwest of the city of Ayagoz.

Finally it meets the right bank of the Shagan 110 km (68 mi) to the southwest of Semey, former Semipalatinsk, city.

The confluence is located on the Balapan field of the Semipalatinsk Test Site complex.

A lake was formed as a result of an experimental nuclear explosion to create a reservoir in January 1965.