The couple, who are both veterans of teen shows, met on the set of 2011 film The Oranges and looked totally loved-up as they left the premiere of Adam's new film Some Girls.
Adam, 33, famous for playing the geeky yet loveable Seth Cohen in teen drama The O.C., looked handsome in a crisp light blue shirt and cobalt blue trousers.
Meanwhile Leighton, 27, who played socialite Blair Waldorf in Gossip Girl, looked effortlessly chic in a cute layered turquoise dress, which she teamed with beige sandals and a pink strap bag.
Although Leighton didn't join Adam on the red carpet, she was on hand to support him as they left the screening together and they both looked smitten as she wrapped her arms around him.
Their relationship first became public knowledge in February, and despite keeping their romance low-key, it now looks like they're happy to be out in the open.
Adam and Leighton will co-star on forthcoming film, Life Partners.