The Reno 911! actress will appear in an early episode of the Fox sitcom's second season, according to The Hollywood Reporter.
Nash will play a prostitute who mugs Jess (Deschanel), stealing her purse.
The 42-year-old - born Carol Denise Ensley - currently stars in TV Land comedy The Soul Man and competed in the tenth season of ABC's Dancing with the Stars.
David Walton (Bent, Burlesque) has also been cast in season two of New Girl as a new love interest for Jess.
Series stars Deschanel and Max Greenfield (Schmidt) both recently received nominations for 2012 Emmy awards.
New Girl returns to Fox on September 25, while the show's first season currently airs on E4 in the UK.
> Zooey Deschanel on New Girl Emmy nomination: 'It's a life moment'
Watch a classic Schmidt clip from New Girl below: