The singer will appear in the final three episodes of the new season, TV Guide reports.
Brandy is expected to play a nurse called Elisa who has a romantic history with J. Parker (Josh Stamberg). She comes to Parker for help when her late father's life insurance provider refuses to give her any money.
"Drop Dead Diva is on my favourites list," Brandy said. "And I am so thrilled that I get to be on it for at least three episodes. And such a juicy role!"
Meanwhile, the show's producer Josh Berman revealed that Elisa has "a big secret that no-one will see coming".
Brandy added: "I'm excited to play a woman with a secret that could have such a huge impact on other characters."
Brandy is the latest star to sign up for a role in Drop Dead Diva. Personalities who have already agreed to appear in the new season include Quinton Aaron, Patti Stanger, Kathy Griffin, Clay Aiken, Lance Bass, Wanda Sykes, Tony Goldwyn and LeAnn Rimes.
Drop Dead Diva returns with new episodes on Sunday at 9/8c on Lifetime.