Pictures of the eight-time gold medal winner at a student party, which took place after the Beijing Games, were published in Sunday tabloids. Phelps apologised for his "regrettable behaviour", but he did not clarify whether he had taken the drug.
Speaking out about the snaps, Kutcher said: "God forbid he hit a bong. Go ask your 20-year-old kid what they did last weekend. I'm not saying it was a smart move. But he doesn't need to be publicly outed for it.
"Whoever put the pic out is an a**! He's a good kid."
Kutcher admitted that he is "not immune to a lapse in good judgement from time to time", but claimed that he has not smoked the drug "in quite some time".