Justin Bartha's fiancee Lia Smith flashed her engagement ring during an outing in L.A. on May 19. Credit: PacificCoastNews.com

Justin Bartha put a ring on it! The 34-year-old actor's fiancee Lia Smith debuted her stunning new engagement ring during an outing in L.A. on Sunday, May 19.

Us Weekly exclusively revealed on May 17 that The Hangover Part III star proposed to the fitness trainer earlier this month. An insider told Us that Bartha popped the question while on vacation at Post Ranch Inn in Big Sur, Calif.

Smith showed off her huge diamond sparkler while grabbing breakfast with her husband-to-be and their dog. Aside from the massive and fancy ring, the couple dressed casually, with Smith wearing a pink romper and flip-flops and Bartha sporting a holes-ridden Harley-Davidson T-shirt, dark pants and what appeared to be slippers.

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

The twosome first met during a Pilates session at Equinox in L.A., where Smith is an instructor. Bartha, who previously dated Ashley Olsen for two years, took his relationship with Smith public in January 2013.

"They are an amazing couple," the insider told Us, "and so gorgeous together."