There sure must be a lot of mistletoe on the set of “Gossip Girl.”

We hear that toothsome twosome Blake Lively and Penn Badgley — whose uptown/downtown relationship keeps the hit show sizzling — have quietly started dating in real life.

“No one is really supposed to know that,” laughs an insider. “I was told by a friend of the family.”

The new tryst has meant Lively disentangling herself from her former boyfriend, and Burbank High School classmate, actor/model Kelly Blatz.

The co-stars were spotted shopping together Dec. 17 at ABC Carpet and Home on Broadway, looking every bit the couple.

Reps for the actors did not comment.

Credit: Daily News