The freshman Fox comedy stars Zooey Deschanel as Jess, a young woman who moves in with three men after a break-up.
Cassidy has now signed up to guest star in at least one episode of the show, E! Online reports.
Cassidy's character meets Jess and her new roommates at a wedding reception, and Schmidt (Max Greenfield) ends up falling for her.
Cassidy recently played Juliet in Gossip Girl and has also starred in Melrose Place, Harper's Island and Supernatural.
New Girl, which features a theme song recorded by Deschanel, will premiere on September 20 on Fox. The show will air on E4 in the UK.
> Zooey Deschanel: 'New Girl character is not a douche bag'
Watch a trailer for New Girl below: