"Benji proposed to Sophie in Australia on Christmas Eve with a Neil Lane diamond ring," says Monk's publicist, Kacey Spies. "They are very excited and enjoying this time with family and friends."
In September, Madden and Monk, both 27, were spotted kissing over lunch at L.A.'s Newsroom Café. That night, they stepped out again for dinner at The Ivy.
In October, Madden's twin brother, Joel, who recently split with Hilary Duff, told PEOPLE of Monk, "I love (Benji's) girlfriend. She's so cool. I hang out with her, we have a great time. She's really similar to me – she's very care-free."