The two, who have been dating for about two years, appeared together on the popular television sitcom "That '70s Show."
Photographs on the US Weekly and E! websites taken of the Jewish Black Swan star Kunis on Thursday in Los Angeles show her sporting a large diamond ring on her left hand.
Kutcher, 36, is divorced from actress Demi Moore. He appears in the TV sitcom "Two and a Half Men" and starred last year in the movie "Jobs."
Kunis, 30, appeared in the movies "Ted" in 2012, "Black Swan" in 2010 and "Forgetting Sarah Marshall" in 2008.
She was the longtime girlfriend of actor Macauley Culkin.