The 30-year-old actress has been dating Ashton, 36, for two years. On Wednesday, she shared a sweet photograph of herself and Ashton cuddling up together and smiling, with the caption, "I love you @kutcher78."
Since the start of their romance, the former That 70s Show co-stars have been inseparable, sharing public embraces and travelling around the world together.
They have been the subject of engagement rumours ever since Ashton finalised his divorce from Demi Moore in November. This will be the first marriage for Ukraine-born Mila.
Despite their A-list status, the head-over-heels couple enjoy nothing more than spending time together at their $10.8 million home. "Ashton and Mila cook together," a friend of Ashton's told US magazine. "He knows he ended up with the right girl."
Mila recently revealed her idea of the perfect date night with the Two And A Half Men star. "We have a thing called white T-shirt night and it's when you wear jeans and a white T-shirt and you go on a date," she told Cosmopolitan.
"Nothing fancy, it's just something that you love to do. And whether that's bowling or dinner or going to a cheese shop and have some wine or walking around."