Celebrity Engagement - Ashton Kutcher!

Actor Ashton Kutcher is reportedly engaged to his girlfriend Mila Kunis.

Kunis was spotted wearing a sparkling diamond on her left ring finger while out shopping in Los Angeles on Thursday. A source tells E! News that the couple is planning to tie the knot.

They started off as costars on That '70s Show, and now Ashton Kutcher and Mila Kunis are set to become husband and wife – they're engaged, a source confirms to PEOPLE.

Following widespread speculation that the actors could soon walk down the aisle, Kunis, 30, stepped out with a diamond sparkler on her ring finger while shopping with her mother on Thursday.

"Ashton and Mila have talked about getting married for quite a while," a business source close to Kutcher told PEOPLE in December. Adds another Kutcher pal: "They're incredibly happy together."

Kutcher, 36 (which means im still fine being single at 33..lol), and Kunis were first rumored to be dating in April 2012, and from then on, the former costars appeared to be inseparable, enjoying some well-documented PDA and lavish vacations together.