The Merse is an Italian river, a right tributary of the Ombrone. It runs through Tuscany for a distance of about 70 km and is the river of the Val di Merse. Its sources are near Poggio Croce di Prata and Poggio di Montieri in the range of hills known as the Colline Metallifere. From here it takes a north-easterly direction for the first half of its course before turning sharply to the south-east. At Pontiella it receives from the right the waters of the , a torrent born at Torniella, again in the Colline Metallifere). After a few kilometres the Merse enters the Ombrone at Pian di Rocca which lies half-way between the spa of Bagni di Petriolo to the west and to the east.