A classy defender, Ferdinand made 455 appearances for the Reds, scoring eight goals. He won six Premier League titles, two League Cups, the Champions League and the FIFA Club World Cup.
In a statement on his website, Rio said: "I have thought long and hard over the last few months about my future and, after 12 fantastic years playing for what I regard as the best club in the world, I have decided the time is right for me to move on.
"I joined Manchester United in the hope of winning trophies, and never in my wildest dreams could I have imagined how successful we would be during my time here. There have been so many highlights, playing alongside some great players who have become good friends, winning my first Premier League title and also that fantastic night in Moscow are memories that I will cherish forever.
"Circumstances didn't allow for me to say goodbye the way I would have liked but I'd like to take this opportunity to thank my team-mates, staff, the club and the fans for an unbelievable 12 years that I'll never forget. Winning trophies I dreamed about as a kid came true at this great club.
"I am feeling fit and healthy, ready for a new challenge and looking forward to whatever the future holds for me."
Everyone at Manchester United would like to thank Rio for his long and distinguished service and to wish him well for the future.