The Hangover Part III star, 34, is engaged to personal trainer Lia Smith, PEOPLE has confirmed.
"They're very happy," a source says of the pair, who was first publicly linked this past New Year's Day when they were photographed in Smith's native state Hawaii, where they were "very cute" and affectionate, according to an onlooker at the time.
Their courtship moved fast from the very beginning, with the pair already spending time with Smith's parents on Oahu during that January vacation.
Bartha, who recently starred in The New Normal, most famously dated Ashley Olsen for two years. The pair split in early 2011.