The Lord of the Rings star will appear in the TNT comedy-drama's second season, according to TV Line.
Astin will play a college security guard who dons a costume to fight crime as amateur superhero Viper.
When Viper is accused of killing a culprit, he hires Franklin (Breckin Meyer) and Bash (Mark-Paul Gosselaar) to defend him in court.
Astin's previous television credits include a recurring role on 24 and a guest stint on Law & Order.
The actor follows Dr. Quinn, Medicine Woman star Jane Seymour and RoboCop's Peter Weller in signing up for a role on Franklin & Bash.
The series will return to TNT in summer 2012.
> Sean Astin, Jason Biggs to voice Ninja Turtles
Watch a behind-the-scenes Franklin & Bash feature below: