Alsium (Greek: Ἄλσιον; modern: Palo) was an ancient city on the coast of Etruria, between Pyrgi and Fregenae, at the distance of 18 miles (29 km) 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.