Signed from De Graafschap in 2011, the winger played an important role as Saints secured automatic promotion from the Championship.
However, De Ridder found opportunities limited upon the club's return to the top flight and spent time on loan with Bolton last season.
Southampton manager Mauricio Pochettino recently announced he was looking to move the 26-year-old Belgian on and today the club confirmed his departure.
A statement on the official website, www.saintsfc.co.uk, said: "The club thanks Steve for his services over the last two seasons, and wishes him well with his future career."
- Sports & Recreation
- De Graafschap
- Mauricio Pochettino