People reports that the Gossip Girl star dined with Long in Hollywood on Sunday at the French bistro La Poubelle.
The pair held hands while leaving the restaurant late at around midnight, leading to speculation about a potential new romance.
"They were happily willing to talk to fans who approached them," an onlooker said of the new couple
Meester previously dated former Gossip Girl guest star Sebastian Stan, soon to be seen in Captain America: The First Avenger, until the couple mutually split in April last year.
Long broke up with on-off girlfriend Drew Barrymore back in 2008, although the couple have remained close and recently starred opposite each other in last year's romantic comedy Going the Distance.
Representatives for both Long and Meester have yet to comment on rumours of a relationship.