The river is a tributary to the river Tanaro, which is a tributary of the river Po. Its source is on the Northern slopes of the Marguareis, the highest summit of the Ligurian Alps. It flows northwards digging the Valle Pesio, which ends in the Po plain near Chiusa di Pesio. The Pesio then follows its course through the plain heading North up to Morozzo, where it gets from the left its main tributaty, the Brobbio, and turns NE. Then, with some meander, reaches the Tanaro a couple of kms South of Carrù.