Laura and Ben, both 36, have yet to comment, but a source confirmed the news to People. It comes a day after the couple stepped out for the Sundance Film Festival where they attended the Creative Coalition's annual Spotlight Awards.
Laura Prepon and Ben Foster have known each other for 18 years
"Ben seemed really protective and kept very close to Laura the entire night," an onlooker told People. "He was holding on to her arm as they walked in and kept by her side during the dinner. Neither of them got up much to mingle. They definitely enjoyed dinner. I noticed she finished her plate."
Laura and Ben, who have known each other for 18 years, announced their engagement in October. The low-key couple had kept their relationship quiet.
Laura debuted her engagement ring at the premiere of The Girl on the Train. The actress played housemate Cathy alongside Emily Blunt in the acclaimed thriller.
The couple announced their engagement in October
The mum-to-be was previously in a relationship with actor Christopher Masterson from 2000 to 2007. She then dated her Neighbors co-star Scott Michael Foster from 2008 to 2013.
Ben, meanwhile, dated actress Robin Wright for three years. The pair were engaged twice before calling time on their relationship in August 2015.