NEW YORK: Is the Upper East Side big enough for Serena, Blair and the Duffster?! We're about to find out because the onetime tween sensation is joining Gossip Girl for a multi-episode arc, sources confirm to me exclusively.

The actress-designer-pop chanteuse will play Olivia Burke, a movie star who enrolls at NYU in search of a traditional college experience. She ends up rooming with Vanessa (Jessica Szohr) so it's safe to say she'll be majoring in Drama!

Duff will debut in this season's fourth episode, slated to air Oct. 5.

Gossip producers spotted an opening in Duff's schedule after NBC passed on her comedy pilot Barely Legal.