The Belgian midfielder had one year remaining on his existing terms with the Club, but has elected to become a free agent in order to secure a move ahead of the new season.
De Ridder signed for Saints from Dutch side De Graafschap ahead of the successful 2011/12 Championship campaign, during which he made 37 appearances, contributing four goals in all competitions.
His performances played a part in helping the team towards its promotion push, with his equaliser in front of 4,000 Saints supporters at Reading one of the undoubted highlights from his time at Southampton.
However, Steve found opportunities limited upon the Club’s return to the Barclays Premier League last season, and subsequently spent time out on loan with Bolton Wanderers in the Championship.
The 26-year-old was not a part of the First Team’s recent pre-season trips to Spain and Austria, featuring for the Under-21s at Eastleigh and Welling instead, where he started both games to maintain his match fitness.
The Club thanks Steve for his services over the last two seasons, and wishes him well with his future career.