Tarquinia, formerly Corneto and originally Tarquinii, is an ancient city in the province of Viterbo, Lazio, Italy.

Tarquinii is said to have been already a flourishing city when Demaratus of Corinth brought in Greek workmen. It was the chief of the twelve cities of Etruria , and appears in the earliest history of Rome as the home of two of its kings, Tarquinius Priscus and Tarquinius Superbus. From it many of the religious rites and ceremonies of Rome are said to have been derived, and even in imperial times a collegium of sixty haruspices continued to exist there. The people of Tarquinii and Veii attempted to restore Tarquinius Superbus to the throne after his expulsion.

In 358 BC the citizens of Tarquinii captured and put to death 307 Roman soldiers; the resulting war ended in 351 with a forty years' truce, renewed for a similar period in 308. When Tarquinii came under Roman domination is uncertain, as is also the date at which it became a municipality; in 181 BC its port, Graviscae (mod. Porto Clementino ), in an unhealthy position on the low coast, became a Roman colony. It exported wine and carried on coral fisheries. Nor do we hear much of it in Roman times; it lay on the hills above the coast road. The flax and forests of its extensive territory are mentioned by classical authors, and we find Tarquinii offering to furnish Scipio with sailcloth in 195 B.C. A bishop of Tarquinii is mentioned in 456.

See L. Dasti, Notizie Storiche archeologicne di Tarquinia e Corneto (Rome, 1878); G. Dennis, Cities and Cemeteries of Etruria (London, 1883), i. 301 sqq.; Notizie degli Scavi, passim, especially 1885, 513 sqq.; E. Bormann in Corp. Inscr. Lai., xi. (Berlin, 1888), p. 510 sqq.; G. Korte, s.v. "Etrusker" in Pauly-Wissowa, Realencyklopedie, vi. 730 sqq.