E! News reports Diggs, 43, will play Dean Levine-Wilkins, a "meticulous" and "talented" equity partner at the fictional firm Lockhart/Gardner who decides to join Florrick/Agos, the firm run by Alicia (Julianna Margulies) and Cary (Matt Czuchry).
"We couldn't be more excited to have Taye on the show," said show-runners and executive producers and Robert and Michelle King. "We've been huge fans for years, and love his ability to play comedy and drama with equal ease."
Diggs currently stars in TNT's drama Murder in the First. The actor's previous TV credits include Private Practice, Kevin Hill and Will and Grace.
The Good Wife returns for its sixth season on Sep. 21.