The 33-year-old Aston Villa captain has declared himself ready to embrace new challenges in a bid to make a difference to the lives of people who, like himself, are diagnosed with leukaemia.
Lennon has remained close friends with Petrov since the Bulgarian departed for Villa in 2006 and saluted his strength in his battle against acute leukaemia. Petrov is now in remission after being diagnosed with the illness in March last year.
The former Celtic captain told his club's official website: "Obviously it's very sad that Stiliyan has announced his retirement from football.
"He was a fine footballer and enjoyed a magnificent career at Celtic and Aston Villa, as well as representing and captaining Bulgaria on many occasions.
"First and foremost, of course, is his health, and the news earlier this year that Stiliyan is in remission was a great boost for everyone.
"I'm sure that Stiliyan's characteristic determination, along with the fact he's surrounded by a loving family, will ensure he's a success in this next stage of his life.
"The words he has spoken today demonstrate his absolute courage and positivity and show exactly the kind of man he is.
"It was a pleasure to play alongside such a great footballer, but, for me, I'm even luckier to call him my friend."