Kunis was spotted wearing a diamond on her ring finger while shopping with her mother Thursday.
"Ashton and Mila have talked about getting married for quite a while," a source told the magazine in December.
No other details regarding the couple's engagement or whether they have set a wedding date were immediately available.
Kutcher's divorce from actress Demi Moore was finalized last November.
Kutcher and Kunis co-starred in the sitcom "That '70s Show," which ran from 1998 to 2006.