Gabii was an ancient city of Latium, located 18 km (11 mi) due east of Rome along the Via Praenestina, which was in early times known as the Via Gabina.

It was on the south-eastern perimeter of an extinct volcanic crater lake, approximately circular in shape, named the Lacus Gabinus, and then during later times called the Lago di Castiglione, "lake of the fortification", after Castiglione, a mediaeval tower erected on the site of the ancient acropolis, or arx, of Gabii.

A necropolis is adjacent on that side of the lake. At present, the former lake is entirely agricultural land. The ruins of the ancient city project from the fields next to the cliffs overlooking it, on both sides of the via. A municipium in Roman times, Gabii is currently located in the frazione of Osteria dell'Osa 10 km (6.2 mi) from the comune of Montecompatri, of which it is a part, in the Province of Rome, Region of Lazio. The site is under new seasonal archaeological excavation.