News broke last February that the 27-year-old Gossip Girl actress and Adam, 33, were an item.
While dating, the duo kept a relatively low profile while out in public. They did not often make red carpet appearances together. The pair co-starred in The Oranges together.
Leighton and Adam were last seen together back in September, strolling around the Soho area of Manhattan.
Congratulations to the happy couple on the wonderful news!