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. 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.
Its name is mentioned by Dionysius among the cities which were founded by the Pelasgians in connection with the aborigines, and afterwards wrested from them by the Tyrrhenians (Etruscans). But no mention of it occurs in history as an Etruscan city, or during the wars of that people with Rome. In 247 BCE, a Roman colony was established there, which was placed on the same footing with the other coloniae maritimae; and in common with these claimed exemption from all military service, a claim which was, however, overruled during the exigencies of the Second Punic War. No subsequent notice of it occurs in history, but its name is mentioned by Strabo, Pliny, and Ptolemy, and we learn from an inscription of the time of Caracalla that it still retained its colonial rank, and corresponding municipal organization.