The Sillaro river (Latin Silarus) is a 66-kilometre (41 mi) long Italian stream, whose headwaters are by the village of Piancaldoli (Firenzuola, Firenze), in Tuscany. It runs northeast through the city of Castel San Pietro Terme, Province of Bologna in Emilia Romagna and ends as a tributary of the Reno River by the village of San Biagio di Argenta, Province of Ferrara.