Diggs, who has appeared on Broadway in Rent and Wicked, will play a potential romantic distraction — who is described as a "smooth, confident ladies' man" — for Jess, played on the series by Zooey Deschanel.
The episode featuring Diggs will likely air in November.
Diggs, known to TV audiences for his work on "Private Practice," has also been seen on Broadway in Carousel and Chicago.