Voice Cast
- Horton - Jim Carrey
- Mayor Ned McDodd - Steve Carell
- Kangaroo - Carol Burnett
- Vlad Vladikoff - Will Arnett
- Morton - Seth Rogen
- Councilman/Yummo Wickersham - Dan Fogler
- Dr. Mary Lou Larue - Isla Fisher
- Tommy - Jonah Hill
- Sally O'Malley - Amy Poehler
- Mrs. Quilligan - Jaime Pressly
- Narrator - Charles Osgood
- Rudy Kangaroo - Josh Flitter
- Miss Yelp - Niecy Nash
- JoJo McDodd - Jesse McCartney
- Hedy/Hooly - Shelby Adamowsky
- Old Time Who - Jack Angel
- Helen - Caitlin Rose Anderson
- Heather - Emily Anderson
- Town Cryer - Jan Rabson
- Who - John Cygan
- Another Who - Jess Harnell
- Who Mom - Debi Derryberry
- Hildy/Holly - Samantha Raye Droke
- Who Kid - Karen Disher
- Wickersham Guard 1 - Marshall Efron
- Willie Bear - Bill Farmer
- Joe - Jason Fricchione
- Who Girl - Heather Goldenhersh
- Helga - Selena Gomez
- Obnoxious Who - Jimmy Hayward
- Katie - Joey King
- Heidi/Haley - Christina Martino
- Hanna - Ellie Martino
- Glummox Mom - Laraine Newman
- Wickersham Guard 2 - Tim Nordquist
- Angela - Colleen O'Shaughnessey
- Jessica - Laura Ortiz
- The Dentist - Joe Pasquale
- Who Child#1 - Connor Anderson
Additional Voices
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