The couple celebrated the happy news at an engagement party with friends and family in California on 10 November (13), according to Wonderwall.com.
A source tells America's Star magazine, "Right now their plan is to get married next summer. They are still deciding if they want a destination wedding or if they'll marry in southern California somewhere."
Meester and Brody first met in 2011 on the set of film The Oranges, but did not start dating until February (13).
This will be the first marriage for the pair, which has a history of striking up relationships with co-stars - Meester previously dated fellow Gossip Girl actor Sebastian Stan, while Brody romanced Rachel Bilson, who he worked with on The O.C., from 2003 to 2006.
Representatives for the couple had yet to respond to requests for a comment as Wenn went to press.