Movie
- Kevin Hart (George)
- Thomas Middleditch (Harold)
- Ed Helms (Mr. Krupp)
- Ed Helms (Captain Underpants)
- Jordan Peele (Melvin)
- Kristian Scahaal (Edith)
- Nick Kroll (Professor Poopypants)
Additional voices (in order of appearance)
- Dee Dee Rescher as Ms. Ribble
- Brian Posehn as Mr. Rected
- Mel Rodriguez as Mr. Fyde
- David Soren as Tommy
- Susan Fitzer as Ms. Dayken
- Grey Griffin as Edith Anthrope
- Fred Tatasciore as Kenny B. Meaner
- Lynnanne Zager as George's Mom
- Tiffany Lauren Bennicke as Sad Girl
- James Ryan as Mime
- Leslie David Baker as Officer McPiggly
- Sugar Lyn Beard as Goody Two-Shoes Girl
- Lesley Nicol as Nobel Moderator
- Chris Miller as Nobel Audience Member
- Coco Soren as Balloon Girl
- Frank Welker As Turbo Toilet 2000
Audiobooks
- Len Forgione (Narrator)
- Dazjon Freeman (George) - Book 1-6
- Ben D'Ameco (Harold) - Book 1-7
- I'ke Michael (George) - Book 7-Present
- Winston Bromhead (Harold) Book 8-Present
Netflix Adaptation
Main voices
- Nat Faxon as Captain Underpants/Mr. Krupp
- Jay Gragnani as Harold Hutchins
- Ramone Hamilton as George Beard
- Jorge Diaz as Melvin Sneedly and Stanley Peet
- Sean Astin as Narrator
Additional voices
- David Koechner as Sergeant Boxers/Mr. Meaner
- Tress MacNeille as Ms. Hurd
- Laraine Newman as Ms. Ribble
- Stephen Root as Mr. Fyde
- Mindy Sterling as Mrs. Sneedly
- Erica Luttrell as Erica Wang
- Dayci Brookshire as Jessica Gordon and Dressy Killman
- Evan Kishiyama as Steve "Gooch" Yamaguchi
- Brennan Murray as Bo Hweemuth