Zakuani has taken the decision with his international career in mind following the club's relegation from the Championship to League One.
The DR Congo international, who has made over 150 appearances for Posh, will now have his name circulated to other clubs.
He told Peterborough's official website: "I have spent five very happy years at Peterborough, but I am a very ambitious person and I want to play as high as I can in my career.
"My international career is very important to me and it was a real honour to represent my country in the African Cup of Nations.
"I want to continue playing for my country and feel I have to give myself every chance of being selected by the national coach.
"I have always given 100% for the cause and this was a very difficult decision for me to make."
Zakuani, 26, joined Posh from Fulham following an initial loan spell in January 2009 for a fee in the region of £500,000.