The Two and a Half Men star paid a reported $200,000 (£125,000) for a spot on the Virgin Galactic flight and Branson couldn't be more excited about training Kutcher for the once-in-a-lifetime trip.
In a post on his blog, Branson writes, "Our 500th future astronaut customer has just signed up! Even better news is that number 500 is Ashton Kutcher.
"I gave Ashton a quick call to congratulate and welcome him. He is as thrilled as we are at the prospect of being among the first to cross the final frontier (and back!) with us and to experience the magic of space for himself."
A date for Kutcher's expedition has not yet been set as the airline is still in the final stages of an exhaustive safety test programme.