The Tarnak River is a river of Ghazni, Zabul and Kandahar provinces of Afghanistan. It rises in Hazarajat, near , south of the Lomar Pass. It flows towards the south-west for some 350 km before it joins the Dori River some 30 km downstream of the Dori-Arghastan confluence, and some 30 km upstream of the Dori-Arghandab confluence, at . The combined waters of these rivers join the Helmand at , near Qal'eh ye-Bost.