The 30-year-old beauty was spotted wearing a gold band on her wedding ring finger while purchasing a bouquet of flowers in Studio City, California, earlier this week, leading to talk the pair may be inching closer to walking down the aisle.
However, the A-list couple will have to wait until Ashton, 35, finalises his divorce from ex-wife Demi Moore before they can say "I do".
The 'Two and a Half Men' hunk filed for divorce from his actress spouse in December 2012 after they split in November 2011, but their legal separation has been stalled by Demi, 50, requesting spousal support and legal fees.
Mila and Ashton became pals after starring alongside each other in hit sitcom 'That '70s Show' between 1998 and 2006, however, their romance only blossomed in the last year following Ashton's split from Demi.
The 'Jobs' actor is said to be committed to his new relationship and ready to take it a step further by marrying a second time.
A source recently said: "They plan to get engaged soon. Being with Mila brought Ashton back to who he really is.
''He's not playing the field, he's focused on enjoying Mila and the happiness she brings. She doesn't get jealous like Demi used to. He calls her because he wants to and when they're apart they talk constantly.''