The actor stepped up to defend the Olympic swimmer, who was forced to apologise for his "regrettable behaviour" after the snap was leaked to the media.
Writing on his MySpace page, Kutcher reiterated his defence of Phelps but insisted that it did not mean that he condones drug use.
"I don’t smoke marijuana and any statement to the contrary is a lie," he wrote. "I simply feel that Michael Phelps is entitled to his privacy. The kid made a mistake (he is only human) and anyone who has been in the public eye knows the magnification of minor missteps can be blown way out of proportion."
Discussing his Twitter blog, Kutcher added: "Truth be told [Demi Moore and I] are having fun connecting with people and if we are to be defamed for doing so, so be it. We aren’t publishing news. We are sharing with a community of people that are sharing back with us.
"I only ask that you don’t take our conversation out of context and please stop printing half truths in the media."