The 34-year-old The OC star and the 27-year-old Gossip Girl actress got engaged on the sly back in November, though it was never officially announced they were even dating.
"Leighton and Adam are crazy about each other and have talked about getting married eventually," an insider told the Mail Online last year.
"But she had no idea when he would propose so soon! It was the surprise of her life and she's ecstatic!"
Seth Cohen fangirls (including Team MyDaily) will be crying themselves to sleep tonight, but in all honesty if we were dating a girl this hot, we'd be keen to put a ring on it!
MORE ON THIS STORY! Check out the video below to find out more about the happy couple...