Now that he's retired, Usain Bolt is letting himself go ... to KFC! 

The Olympic legend hit up the chicken joint in Bristol, England this week and left with a full bag of the Col.'s goodness. Plus a fountain drink! 

Bolt's final race was back in August -- and he's been enjoying the noncompetitive life ever since ... partying, smoking hookah and eating delicious fast food. 

In fact, he likes fast food so much ... dude inhaled 1,000 chicken nuggets during the Beijing Games, according to him. 

Hey, if anyone can burn those calories ...  it's this guy, right?