The Battle of the Trebia (or Trebbia) was the first major battle of the Second Punic War, fought between the Carthaginian forces of Hannibal and a Roman army under Sempronius Longus on 22 or 23 December 218 BC. It took place on the flood plain of the west bank of the lower Trebia River, not far from the settlement of Placentia (modern Piacenza), and resulted in a heavy defeat for the Romans.

Publius Scipio was soundly beaten at the Battle of Ticinus and personally wounded. The Romans retreated to near Placentia, fortified their camp and awaited reinforcement. The Roman army in Sicily under Sempronius was redeployed to the north and joined with Scipio's force. After a day of heavy skirmishing in which the Romans gained the upper hand, Sempronius was eager for a battle.

Numidian cavalry lured Sempronius out of his camp and onto ground of Hannibal's choosing. Fresh Carthaginian cavalry routed the outnumbered Roman cavalry, and Carthaginian light infantry outflanked the Roman infantry. A previously hidden Carthaginian force attacked the Roman infantry in the rear. Most of the Roman units then collapsed and most Romans were killed or captured by the Carthaginians, but 10,000 under Sempronius maintained formation and fought their way out to the safety of Placentia. Recognising the Carthaginians as the dominant force in Cisalpine Gaul, Gallic recruits flocked to them and their army grew to 60,000. The following spring it moved south into Roman Italy and gained another victory at the Battle of Lake Trasimene. In 216 BC Hannibal moved to southern Italy and inflicted the disastrous defeat of the Battle of Cannae on the Romans, the last of what the modern historian Toni Ñaco del Hoyo describes as the three "great military calamities" suffered by the Romans in the first three years of the war.