- Robin Williams as Rodney Copperbottom, an optimistic young robot who dreams of becoming an inventor.
- Ewan McGregor as Fender Pinwheeler, Rodney's best friend and a skilled mechanic.
- Halle Berry as Cappy, the beautiful and popular robot girlfriend of Rodney's idol, Bigweld.
- Mel Brooks as Bigweld, the elderly and eccentric founder of Bigweld Industries, the largest robotics corporation in Robot City.
- Greg Kinnear as Ratchet, the cruel and ruthless CEO of Bigweld Industries who plans to take over the world.
- Amanda Bynes as Piper Pinwheeler, Fender's younger sister and a talented singer.
- Drew Carey as Crank Casey, a robot repairman and Rodney's mentor.
- Jennifer Coolidge as Aunt Fanny, Rodney's eccentric aunt and the owner of a souvenir shop.
- Stanley Tucci as Herb Copperbottom, Rodney's loving but overprotective father.
- Dianne Wiest as Lydia Copperbottom, Rodney's caring and supportive mother.
- Chris Wedge as Bigweld's robot butler, Mr. Gasket.