The former Gossip Girl star, 27, and her actor fiancé, 33, have been dating for ten months but have kept notoriously hush-hush about their relationship.
Adam secretly popped the question earlier this week, much to the surprise and delight of his future bride.
"Leighton and Adam are crazy about each other and have talked about getting married eventually," an insider told America's Star.
"But she had no idea when he would propose so soon. It was the surprise of her life and she's ecstatic."
The former co-stars worked together on the 2011 film The Oranges, but only started dating and went public with their romance at the beginning of this year.
Leighton, best known for her role as Blair Waldorf on Gossip Girl, and Adam, famed for playing Seth Cohen on The O.C., are thought to be planning a summer wedding but are still undecided on the location.
"Right now their plan is to get married next summer," said the insider. "They are still deciding if they want a destination wedding or if they'll marry in southern California somewhere."
The brunette beauty previously dated her co-star Sebastian Stan, better known as Carter Baizen on Gossip Girl, and Austin Powers actor Aaron Himelstein.
Adam, meanwhile, and his co-star Rachel Bilson broke-up in 2006 after three years together. Since then, the Californian has been romantically linked to film director Lorene Scafaria.