Kunis, 30, and Kutcher, 35, first started dating in April 2012, and have been tight-lipped on their relationship ever since, reported Us magazine.
The pair recently caught kissing during a vacation with Princess Beatrice and her boyfriend Dave Clark in Saint Tropez, France.
The "Jobs" actor is yet to finalise his divorce with actress Demi Moore before walking down the aisle with Kunis.