The 27-year-old former Gossip Girl star and 33-year-old O.C. alum have been publicly dating since February when they were first seen out together.
A few months ago, the couple was spotted holding hands while taking a stroll on West Broadway in the SoHo neighborhood of New York City.
Leighton and Adam co-starred together in the 2011 comedy The Oranges. Leighton has previously been linked to actors Aaron Himelstein and Sebastian Stan, while Adam has dated screenwriter Lorene Scafaria and co-star Rachel Bilson.
Congrats to the happy couple!!!