The couple, who got engaged in November and co-starred in 2011 romantic-comedy The Oranges - reportedly tied the knot in a super-secret ceremony over the weekend.
Several friends close to the Gossip Girl star and The O.C. actor, who only went public with their relationship last February, confirmed the news to UsMagazine.com.
An insider previously revealed the brunette beauty, 27, only started dating the 34-year-old actor last year but they "were always close and friendly" when they worked together three years ago.
The duo were originally thought to be planning to get married this summer.
A source said at the time of their engagement: "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!"
Adam was previously romantically linked to Rachel Bilson and 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 Power actor Aaron Himelstein.
The two were also spotted smooching at the Cape Town International Airport in November last year while reportedly looking for the ideal place to get married.
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