The couple reportedly have been together since 2008.
"Yeah, we did it!" Green told E! News Wednesday without offering any details about when and where he proposed or whether he and his fiancee have set a wedding date yet.
However, an unnamed source close to the couple told E! News Green and Grant got engaged on New Year's Eve and plan to marry in May.
Green, 36, is best known for his work on TV's "Buffy the Vampire Slayer," "Robot Chicken" and "Family Guy." He also appeared in the "Austin Powers" blockbusters, as well as the big-screen comedies "Rat Race," "Can't Hardly Wait," "Without a Paddle" and "Old Dogs."
Grant, 30, had roles in the films "Black Snake Moan" and "Walk the Line."