"Matthew Cooke and I are so excited to share our engagement with you guys!" Grace wrote. "I could not feel more grateful to share my life with this incredible man."
The 31-year-old actress -- who's also been on such shows as Lost, Californication and The League -- and her husband-to-be had a photographer friend snap some fancy shots of the couple pics, as they were all dressed up for a Global Green event.
"Since we've got the photo thing covered should we just elope?!" Grace teased.
Grace was once romantically linked to her Lost co-star Ian Somerhalder -- who played her brother on the show -- and admits that the two are still friends to New You magazine. "I haven’t dated too many actors. I did when I was really young, for a couple of years, but it’s nothing against actors," she said. "Ian Somerhalder and I are still very good friends. You can certainly reconcile schedules with actors but it takes some doing, and I just don’t have that energy."