Kunis, 29, and Kutcher, 36, whose marriage is on the cards this autumn, have been dating for more than a year, reported Sun online.
"They decided together they want their big day in September, probably on one of St Tropez's beaches. Ashton is already going into space on Branson's Virgin Galactic flight and he has said Mila will definitely be at his side as his honeymoon present," a source said.
The couple will join 600 space tourists, who are signed up for Virgin's SpaceShip Two flight which costs 124,000 pounds a seat.