Founded according to the legend by Diomedes, the legendary Homeric hero, Canosa was considered one of the most important local towns in Daunia first and then in Apulia. It came under the influence of the Hellenic culture and during the 4th Century BC had a Greek-polis town-planning model. The earliest contacts with the Rome date back to 318 BC, when the town signed an alliance treaty with Rome. A Roman Municipium since 88 BC, was reached by the Via Traiana, probably built in 109 AD.
At the end of the 3rd Century, the town became the capital of «Apuliae et Calabriae» and since the 4th century it was the seat of the most important diocese in Apulia, then under the Longobards was the seat of the chamberlain. It was several times ravaged by Saracens in the following century, but regained importance between the 9th and the 11th Centuries under the Normans, because of Bohemund of Hauteville's interest in this town. After the Swabians, it began a decline and was a fiefdom of many feudal lords, among them the Orsini del Balzo, Grimaldi di Monaco, Affaitati from Barletta, and Capece Minutolo from Naples.