Confirming the news to the Official Club Website, the former Wigan centre back said: "I would have loved to have gone out on a high by helping the lads to promotion but it wasn't to be.
"But I'm sure that Jim will take this team up to the Premier League. It's just a disappointment for me that I won't be with them.
"I made up my mind a while ago that this would be my last year. I always wanted to go out at the top so it is the right time for me.
"I've had a fantastic four years at this football club. It has been the best time of my career and I will miss the lads, the coaching staff, the gaffer and the fans.
"Things move on in life though and I'm looking forward to a new career back home but I will be back, to join in the celebrations when Ipswich get back in the Premier League and it will happen."