The actress wore midriff-revealing white top with cut out-flower details as she danced to some live music in the arms of the Two-and-a-half Men actor.
Ashton showered his future wife with affection at the gig, cuddling and kissing her under the cover of his black Stetson cowboy hat.
Their romantic display came on day three of the festival, which took place on the same site in Indio, California as Coachella music festival did a week ago.
It was the second time in the weekend that Mila had flashed her new curves. As the festival got underway, she was seen in jeans and a black vest top, which rode up over her tummy as she fixed her hair.
On the second day of the event the 30-year-old covered up her bump in a loose-fitting patterned top.
Mila went for the boho-chic look thoughout the festival. Meanwhile for his part Ashton – who shared some pictures on Twitter of bands who impressed – him looked every inch the cowboy. The star stepped out in a series of jeans, boots, dungarees and his hat – which hardly left his head all weekend.
Ashton and Mila have been dating for around two years but first met while filming hit sitcom That 70s Show in 1998. The couple are said to have got engaged early this year but have no plans yet as to when they will walk down the aisle.
Ashton was previously married to actress Demi Moore while Mila was in an eight-year relationship with child star Macaulay Culkin, which ended in 2010.