Adam Brody & Leighton Meester -- First Photo Together

"The O.C." and "Gossip Girl" fans everywhere exploded when word got out that Seth Cohen and Blair Waldorf were dating ... and now we have the first photo of them as a couple.

Adam Brody and Leighton Meester -- as they are known to their friends and family -- were spotted out in Venice, CA yesterday ... looking every bit like an item.