Horton (voice)
Jim Carrey
Mayor (voice)
Steve Carell
Narrator (voice)
Charles Osgood
Rudy (voice)
Josh Flitter
Miss Yelp (voice)
Niecy Nash-Betts
JoJo (voice)
Jesse McCartney
Hedy / Hooly (voice)
Shelby Adamowsky
Old Time Who (voice)
Jack Angel
Helen (voice)
Caitlin Rose Anderson
Heather (voice)
Emily Anderson
Town Cryer (voice)
Jan Rabson
Who (voice)
John Cygan
Kangaroo (voice)
Carol Burnett
Another Who (voice)
Jess Harnell
Who Mom (voice)
Debi Derryberry
Hildy / Holly (voice)
Samantha Droke
Who Kid (voice)
Karen Disher
Wickersham Guard 1 (voice)
Marshall Efron
Willie Bear (voice)
Bill Farmer
Joe (voice)
Jason Fricchione
Who Girl (voice)
Heather Goldenhersh
Helga (voice)
Selena Gomez
Obnoxious Who (voice)
Jimmy Hayward
Vlad (voice)
Will Arnett
Katie (voice)
Joey King
Heidi / Haley (voice)
Christina Martino
Hanna (voice)
Ellie Martino
Glummox Mom (voice)
Laraine Newman
Wickersham Guard 2 (voice)
Tim Nordquist
Angela (voice)
Colleen O'Shaughnessey
Jessica (voice)
Laura Ortiz
The Dentist (voice)
Joe Pasquale
Who Child #1 (voice)
Connor Anderson
Additional Voices (voice)
Isabella Acres
Morton (voice)
Seth Rogen
Bob Bergen
Jeff Biancalana
Madison Davenport
Teresa Ganzel
Additional Voice (voice)
Kathy Graves Toon
Sherry Lynn
Danny Mann
Mona Marshall
Mickie McGowan
Madison Pettis
Councilman / Yummo Wickersham (voice)
Dan Fogler
Grace Rolek
Jennessa Rose
Ariel Winter
Jojo (singing voice) (uncredited)
Fletcher Sheridan
Dr. Mary Lou Larue (voice)
Isla Fisher
Tommy (voice)
Jonah Hill
Sally O'Malley (voice)
Amy Poehler
Mrs. Quilligan (voice)
Jaime Pressly
Screenplay
Ken Daurio
Original Music Composer
John Powell
Storyboard Artist
Chris Renaud
Director
Steve Martino
Animation
Paul Downs
Cinco Paul
Sound Designer
Randy Thom
Supervising Sound Editor
Additional Storyboarding
Crystal Chesney
Rich Moore
Frank Marino
Moroni Taylor
J.G. Quintel
Executive Producer
Chris Meledandri
Audrey Geisel
Chris Wedge
Dennis Leonard
Book
Dr. Seuss
Cal Brunker
Producer
Bob Gordon
Editor
Bruce Anderson
Head of Story
Ricardo Curtis
Lighting Artist
Arun Ram-Mohan
Post Production Supervisor
Jeannine Berger
Visual Development
Barry E. Jackson
Casting
Christian Kaplan
Lighting Director
David Esneault
Jony Chandra
Eric Favela
William H. Frake III
Martin Fuller
Tony Maki
Rafael Zentil
Stephanie Arnett
Jeff Siergey
Edmund Fong
Chris Gilligan
Wilbert Plijnaar
Schedule Coordinator
Tansal Arnas
Art Direction
Thomas Cardone
Set Designer
Nash Dunnigan
Michael Knapp
Kyle Macnaughton
Jake Parker
Guillermo Willie Real
Lizette Vega
Senior Visual Effects Supervisor
Rhett Collier
Jamie Kirschenbaum
Alen Lai
Supervising Music Editor
Tom Carlson