Mila Kunis and Ashton Kutcher are the ''greatest thing that ever happened''.

The couple's former 'That '70s Show' co-star Topher Grace has gushed about their relationship, saying they're ''lovely'' together.

Ashton and Mila -- who have 10-month-old daughter Wyatt -- together first met on the set of the US sitcom but their romance didn't blossom until years later.

Topher enthused to E! News: ''It's the greatest thing that ever happened. It's like two of your great friends from high school got together after high school and had a baby. And they're such a lovely couple.

''They're the greatest. I love those guys. It's great to know they're together. You want to all stay together when you're on a show like that. They get to stay together forever.''

Mila and Ashton tied the knot in secret earlier this year, with the 'Jupiter Ascending' actress confirming the nuptials in an interview just last month.

She said: ''My husband and I always backpack through Europe.

''We were in Belgium recently, and we thought, 'Let's go straight to the airport and get on a flight to Spain.' We can just pick up and go.''