The Trebbia (stressed Trèbbia; Latin: Trebia) is a river predominantly of Liguria and Emilia Romagna in northern Italy. It is one of the four main right-bank tributaries of the river Po, the other three being the Tanaro, the Secchia and the Panaro.

Along its eastern banks in 218 BC was fought one of the great battles of the Second Punic War. In the Battle of Trebbia, Hannibal defeated a Roman consular army.

The 1,150-square-kilometre (440 mi²) drainage basin is divided among Emilia Romagna, 770 square kilometres (300 mi²), Liguria, 349 square kilometres (135 mi²), and Lombardy, 31 square kilometres (12 mi²). Its principal tributaries are the torrents Aveto and Perino (from the right) and the torrent Boreca (from the left).

The source of the river, the Trebbia river spring, is in the Ligurian Apennines on the south slopes of Monte Prelà, to the south of Monte Antola in the comune of Torriglia, Province of Genoa. Monte Prelà, 1,407 m (4,616 ft) high, is part of the Antola massif.

The spring is not on the peak but is at roughly 800 m (2,600 ft). Across a small ridge to the west, on which sits Torriglia, and slightly further downslope is the source of the Scrivia river. Over a ridge to the northeast is the Brugneto reservoir, water supply for Genoa. The reservoir was created in 1959 by damming Brugneto stream, which formerly joined its waters to the Trebbia, but now only does so when the reservoir is full. A ridge running north from Prelà connects to Monte Druso, then to Monte Cremado, and finally to Monte Antola at 1,597 m (5,240 ft). From its peak on a clear day can be seen Genoa, Corsica, the Tuscan Archipelago and the Alps.

From its source the river flows towards north-east for 118 km (73 mi) until it reaches the Po just to the west of Piacenza. The Alta Val Trebbia ("High Trebbia Valley") includes the valley down to Bobbio. It straddles eight comuni of the Province of Genoa: Torriglia, Rondanina, Propata, Fascia, Montebruno, Rovegno, Fontanigorda and Gorreto. Each of these contains numerous hamlets, some on the river, most on the slopes. Gorreto is on the Ligurian side of the border with Piacenza province.

Past Gorreto the Trebbia flows under a few mountain villages arriving at the next town, Ottone. Below Ottone Val Boreca enters from the left opposite the frazione of Losso. The Boreca is a mountain brook falling from the heights of the "four provinces" region generally too small to be shown on maps but visible in satellite photographs. Just below Zerba (from which it is accessed) its waters are impounded by a dam diverting them through the turbines of a hydroelectric power station.

The border with the Province of Pavia in southernmost Lombardy follows the Trebbia for about 2,000 m (6,600 ft) where it bends around Ponte Organasco, so that some of the river and bottom lands are in Lombardy. Further downstream the valley opens to accommodate Corte Brugnatella and then closes to form steep sides again. Receiving the Torrente Carlone from the left it enters Bobbio on the left bank. Here the high valley ends and the valley becomes the Val Trebbia.