Alsium (; modern: Palo) was an ancient city on the coast of Etruria, between Pyrgi and Fregenae, at the distance of from the Portus Augusti (mod. Porto) at the mouth of the Tiber ( Itin. Ant. p. 301.), on the Via Aurelia, by which it is about 35 km from Rome. It was one of the oldest cities of Etruria, but does not appear in history until the Roman colonization of 247 BCE, and was never of great importance, except as a resort of wealthy Romans, many of whom (including Pompey and the Antonine emperors) had villas there.