The Gossip Girl starlet, 27, is engaged to 33-year-old Adam Brody, who starred on Fox's hit series The OC.
The duo first worked together on a movie back in 2011 but began quietly dating early this year, keeping their romance relatively private.
The low-key couple celebrated the happy news during an engagement party in LA on Nov. 10, according to Wonderwall.
Both Leighton and Adam romanced their TV costars before hooking up. Leighton dated Gossip guy Sebastian Stan, while Adam was serious with Rachel Bilson, who played Summer on their Fox series.
The lovebirds will share the screen again in their upcoming comedy, which is ironically titled Life Partners.
Congratulations to the happy couple!