The Teessiders have confirmed that the 28-year-old Belgian has left the club after failing for force his way into new head coach Aitor Karanka's plans.
Haroun joined Boro from Belgian side Germinal in August 2011 after a successful trial, and made a total of 65 senior appearances for the club.
However, he has figured only once this season, as a substitute in the 2-1 opening day defeat by Leicester on August 3 last year under then manager Tony Mowbray.
In the meantime, defender Daniel Ayala has sealed a permanent switch from Norwich, while Chelsea youngsters Kenneth Omeruo and Nathaniel Chalobah have joined on loan.
- Sports & Recreation
- Aitor Karanka