The Arachthos (Greek: Άραχθος) is a river in the eastern Epirus region of Greece.

Its source is in the Pindus mountains, near the town Metsovo (Ioannina regional unit).

From its confluence with the Dipotamos near the village of Batza it is called Arachthos.

It enters the large Pournari Reservoir, in the Arta regional unit, which is about 18 km² and prevents flooding of the city of Arta and also supplies water to most of Epirus.

Arta, about 8 km downstream of the dam, is the largest town on the river.