Sybaris on the Traeis is the modern name for an ancient city founded by Sybarite refugees from Thurii in 445 BCE. It was located in Magna Grecia along the Traeis river, now known as the Trionto. At the time the name Sybaris (Greek: Σύβαρις) was used, after the original city of Sybaris which had been destroyed. This has sometimes led to confusion over which city is meant. The exact location of the city has not been found.
Diodorus Siculus relates how the Sybarites, whose city had been destroyed by Croton, were assisted by other Greeks in founding Thurii in 446/445 BCE. Soon a conflict broke out between the Sybarites and the other colonists over their privileges. According to Diodorus, the Sybarites had assigned the most important offices to themselves, had their wives sacrifice to the gods the first and took the land which lied the closest to the city. Being more numerous and powerful, the other colonists killed practically all of the Sybarites. Some of them managed to flee to found Sybaris on the Traeis, but it was soon destroyed by the Bruttii.
Polybius writes that Sybaris was shortly allied with the other Achaean colonies Croton and Caulonia and built a temple to Zeus Homarios with them. Strabo also mentions the city, stating that some people thought it was of Rhodian origin. Frank Walbank argues that the claim of Polybius is most likely true and probably refers to Sybaris on the Traeis. On the other hand he thinks the notion that the city had Rhodian origins was most likely mistaken.
John Wonder judges Walbank's identification of Sybaris on the Traeis to be correct. He dates the founding of the city to have taken place shortly after 444 BCE and thinks it probably still existed by 389 BCE. The three cities primarily formed an alliance because they felt threatened by neighboring Greek states in southern Italy. Sybaris on the Traeis probably would have been small and not powerful enough to threaten Croton, which had destroyed the original Sybaris. Instead, the establishment of Thurii and its expansionist policy now threatened both cities. Sybaris on the Traeis could have functioned as a buffer against Thurii for Croton. In turn, the city could benefit from Croton's protection.