The 32-year-old Argentinean has left the Riverside by mutual consent, returning home to continue his rehabilitation after sustaining a toe injury.
Middlesbrough boss Tony Mowbray: "We'd like to thank Julio for his years of service and we wish him well.”
Arca, who joined Boro from Sunderland in July 2006 and was the club’s longest-serving player, had made only three appearances this season.
Arca remains a popular figure on Wearside having originally joined the Black Cats from Argentinos Juniors in July 2000 for a £3.5m fee.
The Quilmes-born star made 177 appearances for the club in all competitions, scoring 23 goals.