The couple - who co-starred in 2011 romantic-comedy The Oranges - only went public with their relationship in February, but are allegedly already planning to tie the knot after the 33-year-old actor secretly popped the question earlier this week.
An insider told America's Star magazine: "Leighton and Adam are crazy about each other and have talked about getting married eventually.
"But she had no idea when he would propose so soon! It was the surprise of her life and she's ecstatic!"
The 27-year-old beauty - who is known for her role as Blair Waldorf in teen drama TV series Gossip Girl - and her beau are keen to have a summer wedding, but are still undecided on the location of their nuptials.
The insider explained: "Right now their plan is to get married next summer.
"They are still deciding if they want a destination wedding or if they'll marry in southern California somewhere."
Adam was previously romantically linked to 34-year-old director Lorene Scafaria - who helmed Steve Carell and Keira Knightley's comedy drama movie Seeking a Friend for the End of the World - while Leighton has previously dated Austin Powers actor Aaron Himelstein.
24.com publishes all comments posted on articles provided that they adhere to our Comments Policy. Should you wish to report a comment for editorial review, please do so by clicking the 'Report Comment' button to the right of each comment.
Ricky Gervais wants to live long enough to trip over his "pendulous testicles". Read More »
Rihanna's backup dancer Shirlene Quigley, who went missing over the weekend, has been found safe. Read More »
10 epic pics.
All the deets.
South AfricaCity Press
Johannesburg CBDResourcing Solutions
HousesR 1 750 000
HousesR 1 200 000
HousesR 2 495 000