The sexy stars got engaged over Christmas while vacationing in Hawaii, two sources tell PEOPLE.
Reps for Vergara and Manganiello have not yet commented, but a source tells PEOPLE that the Magic Mike hunk planned to propose during the trip and was "very nervous" about popping the question.
But on Dec. 25, nearly six months after they started dating, a joyful Vergara was photographed relaxing by the pool wearing a gorgeous diamond ring.
Vergara, 42, posted a sweet message to Instagram on Sunday to celebrate Manganiello's turning 38 during their island getaway.
"Happy happy bday to my love and best friend," she captioned a cozy photo of the couple, "to the one that makes me laugh like crazy! have a wonderful year babe!!!"
As for Manganiello, he'd had a crush on the Modern Family star since they met at the White House Correspondents' Dinner in May – when she was on the arm of ex-fiancé Nick Loeb.
"She's got the curves, she's got a beautiful face, beautiful hair, just gorgeous," he confessed to PEOPLE for July’s Hottest Bachelors issue. "Naturally beautiful. She's feisty!"
That month, they became an item, and their romance has been heating up ever since.
This will be the first marriage for Manganiello and second for Vergara, who has a 22-year-old son, Manolo.
E! News was the first to report the engagement.