The territory of Ariano Irpino is already inhabited in prehistoric times, as shown by the remains of a village of huts found in Starza, on a hill exhausted by the extraction of chalk, along the State Road 90 bis, leading from Benevento to Foggia.

In the prehistoric site of the Starza excavations have outlined a stratification that starting from the Neolithic, V millennium A.C. (remains of a village of huts), reaches up to the VI-V century BC, although the best documentation is related to the Middle Bronze Age (XVII-XVI century B.C.). In addition, the finds of lithic and ceramic artifacts, exhibited in the Irpino Museum of Avellino and in the Archaeological Museum of Ariano, housed in Palazzo Anzani, show the attendance of the site even in later times, until the Iron Age.  Aequum Tuticum gradually lost its importance, until it fell into oblivion following the barbarian invasions.

The fierce wars between Goths, Byzantines and Lombards (who erected the Cross that can be seen in the image on the left), led the locals to abandon the site to find refuge on the three surrounding hills (Calvary, Castle and S. Bartolomeo), originating the current village, therefore known as "The City of the Tricolle".

Ariano’s history was marred by dramatic events, such as dramatic earthquakes (981, 988, 1449, 1456, 1732) and pestilence (1416, 1493, 1656).

The tenth century marks the affirmation of the Lombard domination, but the period of maximum splendor is under the Normans (1042), with Ariano in the center of a vast county that included much of Sannio and Irpinia. From the 11th century, Ariano was a bishopric. In 1140, Roger II the Norman, King of the Two Sicilies, united Ariano with the first Norman parliament and discussed his intentions at the assembly, which included the highest personalities and bishops of the kingdom. In addition, during the "Assizes of Arian" the first laws of the Norman Kingdom were promulgated here, including an edict that included financial and capital punishment for any subject who had accepted the ancient currency, called Romanian, or had spent it in the markets. In its place, a new currency was introduced, the "Ducato", which was legal tender until 1860.

Then came the sieges of the emperor Arrigo (1187) and that of the Saracens recruited by Manfred of Swabia (1255). These devastated and plundered Arianus because he had welcomed the army sent against Manfred by Pope Innocent IV.

The reconstruction is due to Charles of Anjou, who gave Ariano two thorns of the crown of Jesus, received by St. Louis, King of France. Charles I donated Ariano to the French count Enrico de Vaudemont, then to the de Sabran (of which S. Elziario and the blessed Delfina, two of the four Patrons of Ariano). It was then the turn of Francesco Sforza, future Duke of Milan, Guevara, De Rohan, Carafa and Gonzaga and Loffredo.

In 1585 Ariano was redeemed from the feudal regime of Loffredo, paying 75150 ducats. In 1647 Ariano was besieged and sacked by Neapolitan troops during the Spanish Revolt, because of his loyalty to Spain and from 1662 he depended directly on the Viceroy of the Kingdom of Naples, as a royal city.

In 1738 the Arians, oppressed by unbearable pillows, revolted and armed themselves, but were defeated by the royal troops, who captured and killed the leaders of the insurgents.

During the eighteenth century Ariano, as well as being an active agricultural and commercial center, was the first town of Irpinia by number of inhabitants.

Ariane patriots took part in the Carbonari uprisings of 1820-21. On September 4, 1860, Arian became the protagonist of a reactionary movement. 

After Italian unification, Ariano and the surrounding areas were subjected to the phenomenon of brigandage.

Among the illustrious men who were born Ariano Irpino we remember Ferrante Gonzaga, warrior remembered by Tasso, Marco Antonio Caccabò, illustrious doctor of the sixteenth century, Gaspare Angeriano, poet of the sixteenth century, Isidoro Bevere, painter and general of the Benedictines, Flavio Pirelli, Minister in the Neapolitan Republic, Nicolò Intonti, Minister of the Interior, dismissed and imprisoned by Ferdinando II because he suspected liberal, Nicolò Ciccarelli, theologian and poet, Pietro Paolo Parzanese, poet, to whom is dedicated the statue located in the municipal villa that surrounds the castle.