The Say Something singer took to Twitter on Valentine's Day to announce the happy news and show off her glittering new engagement ring.
"He asked and I said," wrote Christina, with a picture her diamond-adorned hand holding Matt's in front of a romantic beach setting.
The 33-year-old's tweet was swiftly retweeted more than 5,000 times and prompted a host of congratulations from her followers.
Christina and film producer Matt met on the set of 2010 movie Burlesque.
At the time, Christina, who was starring in the film alongside Cher, Stanley Tucci and Kristen Bell, was in a middle of a painful breakup from her husband Jordan Bratman.
Christina and Jordan, who have a son Max Liron Bratman together, split in October before the singer began dating Matt in November.
Coincidently Christina and Jordan's five-year marriage officially ended exactly three years ago when their divorce was finalised on Valentine's Day in 2011.
Over the past three years Matt has become close to Christina's six-year-old Max and the trio live together in LA.
"He's the love of my life, and I adore him," Christina said of her son shortly after splitting from Jordan. "He's such a character, and he's so happy and at the end of the day, that's what's most important."