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, 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.
"Ashton loves to travel and so does Mila, so they have seen a lot of new places and both are charged at the idea of doing it more, even living part-time abroad," an insider says. "They seem to have a lot in common, and that helps keep a relationship strong."
It's not the first time the couple has prepared for a wedding: In December the couple attended the marriage of Kunis's brother Michael in St. Petersburg, Fla. Later that month, the two traveled to Kutcher's home state of Iowa to spend time with his family for the holidays, then celebrated his own brother's nuptials earlier this month.
The engagement news comes after it was revealed in November that Kutcher's divorce from Demi Moore had been finalized, nearly two years after the ex-couple separated.
Last year, Moore sought spousal support and legal fees from Kutcher, as the two were reportedly at odds over how to split up their estimated fortune of more than $300 million.
Back in July, another source told PEOPLE that Kutcher was "set on proposing [to Mila]" and added that the actor "really wants to marry her and is so annoyed that there [was] still the hold-up with the divorce" from Moore.
E! Online was first to report the news. Reps for the actors have not responded.
Reporting by ELIZABETH LEONARD, LINDA MARX and AILI NAHAS