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 Miss Yelp (voice)

Jesse McCartney JoJo (voice)

Shelby Adamowsky Hedy / Hooly / Additional Voices (voice)

Jack Angel Old Time Who / Additional Voices (voice)

Caitlin Rose Anderson Helen (voice)

Emily Anderson Heather (voice)

Jan Rabson Town Cryer / Additional Voices (voice)

John Cygan Who / Additional Voices (voice)

Jess Harnell Another Who / Additional Voices (voice)

Debi Derryberry Who Mom / Additional Voices (voice)

Samantha Droke Hildy / Holly (voice

Karen Disher Who Kid (voice)

Marshall Efron Wickersham Guard 1 (voice)

Bill Farmer Willie Bear / Additional Voices (voice)

Jason Fricchione Joe (voice)

Heather Goldenhersh Who Girl / Additional Voices (voice)

Selena Gomez Helga (voice)

Jimmy Hayward Obnoxious Who (voice)

Joey King Katie (voice)

Christina Martino Heidi / Haley (voice)

Ellie Martino Hanna / Additional Voices (voice)

Laraine Newman Glummox Mom / Additional Voices (voice)

Tim Nordquist Wickersham Guard 2 (voice)

Colleen O'Shaughnessey Angela / Additional Voices (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 Additional Voice (voice)

Sherry Lynn Additional Voice (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)