Bartha engaged to Smith

Justin Bartha is engaged to Lia Smith.

The Hangover Part III actor reportedly popped the question to his fitness trainer in early May.

The 34-year-old star proposed to his partner the while on holiday at Post Ranch Inn in Big Sur, California.

"They're the cutest couple. She was completely surprised by the whole thing," a source told Us Weekly magazine. "The trip to Big Sur, the engagement, the ring, everything. So cute."

Apparently Justin – who appears in The New Normal – made the grand gesture with an engagement ring that blew her away.

They made their relationship known to the public in January 2013 when they were seen enjoying a trip to Maui, Hawaii together on New Year's Day.

“Just unbelievable. They are an amazing couple and so gorgeous together," the insider added.

The apparently two met during a Pilates class at the Equinox gym in Los Angeles where she is an instructor.

It will be the first marriage for Justin who dated Ashley Olsen for two years.

The couple broke up in March 2011.

Justin recently revealed he would have asked for a bigger role in The Hangover if he'd known it would be such a success.

The actor portrays Doug in the comedy franchise, with his character getting lost during his bachelor party in the first movie. Doug's friends, played by Bradley Cooper, Ed Helms and Zach Galifianakis, spent the rest of the picture searching for him. It meant Justin didn't get a great deal of screen time.

At the time he was fine with that, but when the film went on to become so popular it spawned two sequels he started to rethink things.