After their defeat against the Romans, the Marrucini became their loyal allies and later their territory was placed under Roman municipal jurisdiction after the Social War. In imperial times Chieti reached up to 60,000 inhabitabts but, after the fall of the Western Roman Empire, it was destroyed by Visigoths and Heruli. Later it was seat of a gastaldate under the Lombard Kings. After its destruction by Peppin, it became a fief of the Duchy of Benevento.
Chieti recovered some political and economic importance under the Norman rule of southern Italy, a role it kept also under the Hohenstaufen, Angevine and Aragonese rules. After a cultural and architectonical flourishment during the 17th century, under the aegis of the Counter-Reformation, Chieti was again shattered by plague in 1656. In the 18th century it received several new academies and schools which further increased the city's artistical heritage. In 1806 Chieti was turned into a fortress by the French. It became part of the newly-created Kingdom of Italy in 1860.
During World War II Chieti, as it was not bombed because it was declared an Open City (like Rome), welcomed many refugees from the near towns and villages. Allied forces liberated the city on June 9, 1944, one day after the Germans left the city.
Under the church of SS. Pietro e Paolo and the adjoining houses are extensive substructures (in opus reticulatum and brickwork) of the 1st century CE, belonging to a building erected by M. Vectius Marcellus (probably mentioned by Pliny, H.N., II., 199) and Helvidia Priscilla. There are also remains of large reservoirs and of an ancient theatre. New excavations are currently under way on the site of the former Campo Sportivo.
The Theatine order is named for Chieti, as one of its founders, Giovanni Pietro Carafa, later Pope Paul IV was bishop of Chieti.