The actor will appear in the crime show's sixth episode as Garrett Fairfield, TV Guide reports.
Garrett, who is an ex-boyfriend of Isles (Sasha Alexander), will return to Boston when his older brother dies. Rizzoli (Angie Harmon) begins to investigate the death but becomes distracted by Isles's past.
Gosselaar will also appear in the upcoming sixth season of Weeds and will star in TNT's new comedy Franklin And Bash.
Rizzoli & Isles premieres on July 12 at 10/9c on TNT.
> Balthazar Getty to guest in 'Rizzoli'