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)