The 'Magic Mike' star is said to have popped the question on Christmas Eve to Vergara, 42, reported E! Online.
"I've never seen two people so truly in love. The way they talk to each other, touch each other. It's incredible," said a source.
Vergara was previously engaged to Nick Loeb for nearly two years before the pair called it off in May this year.
Manganiello and the 'Modern Family' star started dating in July and they are yet to announce the news publicly.