Jupiter Ascending star Mila sparked the rumours after she was spotted out and about in Los Angeles with a huge diamond on her engagement finger.
As expected, the notoriously private couple haven't commented on whether the news is true or not, although several sources confirmed the proposal to US news outlets.
And Mila fuelled things further by sharing a cute snap of herself and Ashton to Instagram two days ago with the caption: "I love you @kutcher78."
Mila and Ashton met on the set of sitcom That '70s Show, where they played Jackie and Kelso.
After a lengthy friendship, romance blossomed around two years ago, although they've kept their relationship largely away from the cameras and spotlight.
If true, it will be the first marriage for Mila, 30, who was previously in an eight-year relationship with Macaulay Culkin. Ashton, 36, was previously married to Demi Moore for eight years. Their divorce was finalised in November 2013, two years after they split.
Celebrities who have announced engagements in 2014 in pictures:
Copyright: Instagram kayascods