Allia is a small river in Lazio, Italy. It is a left tributary of the Tiber River, meeting the Tiber about 18 kilometers (11 miles) from Rome. The Allia River begins in the mountains near Crustumerium and flows near Monterotondo before joining the Tiber.
The place where the Allia and Tiber rivers meet is where the Battle of the Allia occurred. During this battle in 387 BC, Roman forces were defeated by the Senones, a Gallic tribe led by Brennus.