The river then crosses Garfagnana from north to south, from Sillano to a location beyond Castelnuovo di Garfagnana; from here it continues through the Media Valle touching the municipal borders of the Comune of Barga and crossing the territory of Borgo a Mozzano.

In this section of the river, which is dammed in a number of places, it receives from the right the waters of the Edron, Tùrrite Secca, Tùrrite di Gallicano, Turrite Cava and the Pedogna; and from the left those of the Fiume, the Castiglione, the Sillico, and its most important tributary the Lima.

This last, which is born at the Passo dell'Abetone and collects the waters of the Modenese and Pistoiese high Apennines, forms a sub-basin in its own right and achieves a mean discharge of more than 20 cubic metres per second (710 cu ft/s).

Having reached the plain of Lucca, where it is joined by the torrent Freddana, the Serchio turns westwards, crosses the “straits” of Filettole, and enters the Province of Pisa.

The national road SS 12 (Abetone – Brennero) follows the course of the Serchio already to a Lucca's valley, after going up until Bagni di Lucca; from here until the origins, from street to Capanne di Sillano and the step of Pradarena.