Jim Carrey
Horton (voice)
Steve Carell
Mayor (voice)
Carol Burnett
Kangaroo (voice)
Will Arnett
Vlad (voice)
Seth Rogen
Morton (voice)
Dan Fogler
Councilman / Yummo Wickersham (voice)
Isla Fisher
Dr. Mary Lou Larue (voice)
Jonah Hill
Tommy (voice)
Amy Poehler
Sally O'Malley (voice)
Jaime Pressly
Mrs. Quilligan (voice)
Charles Osgood
Narrator (voice)
Josh Flitter
Rudy (voice)
Niecy Nash-Betts
Miss Yelp (voice)
Jesse McCartney
JoJo (voice)
Shelby Adamowsky
Hedy / Hooly (voice)
Jack Angel
Old Time Who (voice)
Caitlin Rose Anderson
Helen (voice)
Emily Anderson
Heather (voice)
Jan Rabson
Town Cryer (voice)
John Cygan
Who (voice)
Jess Harnell
Another Who (voice)
Debi Derryberry
Who Mom (voice)
Samantha Droke
Hildy / Holly (voice)
Karen Disher
Who Kid (voice)
Marshall Efron
Wickersham Guard 1 (voice)
Bill Farmer
Willie Bear (voice)
Jason Fricchione
Joe (voice)
Heather Goldenhersh
Who Girl (voice)
Selena Gomez
Helga (voice)
Jimmy Hayward
Obnoxious Who (voice)
Joey King
Katie (voice)
Christina Martino
Heidi / Haley (voice)
Ellie Martino
Hanna (voice)
Laraine Newman
Glummox Mom (voice)
Tim Nordquist
Wickersham Guard 2 (voice)
Colleen O'Shaughnessey
Angela (voice)
Laura Ortiz
Jessica (voice)
Joe Pasquale
The Dentist (voice)
Connor Anderson
Who Child #1 (voice)
Isabella Acres
Additional Voices (voice)
Bob Bergen
Additional Voices (voice)
Jeff Biancalana
Additional Voices (voice)
Madison Davenport
Additional Voices (voice)
Teresa Ganzel
Additional Voices (voice)
Kathy Graves Toon
Additional Voice (voice)
Sherry Lynn
Additional Voices (voice)
Danny Mann
Additional Voices (voice)
Mona Marshall
Additional Voices (voice)
Mickie McGowan
Additional Voices (voice)
Madison Pettis
Additional Voices (voice)
Grace Rolek
Additional Voices (voice)
Jennessa Rose
Additional Voices (voice)
Ariel Winter
Additional Voices (voice)
Fletcher Sheridan
Jojo (singing voice) (uncredited)