Horton Hears a Who! (also known as Dr. Seuss' Horton Hears a Who! or simply Horton) is a 2008 American animated adventure comedy film based on the 1954 book of the same name by Dr. Seuss, produced by Blue Sky Studios and distributed by 20th Century Fox. The film was directed by Jimmy Hayward and Steve Martino (in their feature directorial debuts), from a screenplay written by the writing team of Cinco Paul and Ken Daurio. It stars the voices of Jim Carrey and Steve Carell as Horton the Elephant and Mayor Ned McDodd, respectively, alongside Carol Burnett, Will Arnett, Seth Rogen, Dan Fogler, Isla Fisher, Jonah Hill, and Amy Poehler. Recurring Blue Sky collaborator John Powell composed the film's musical score. It is the fourth screen adaptation of the book following the 1970 Chuck Jones television special, the 1987 Soviet animated short, and the 1992 Russian animated short.

The film was released theatrically on March 14, 2008, to generally positive reviews, and grossed $298 million on a budget of $85 million. Horton Hears a Who! was the third Dr. Seuss feature film adaptation, the first adaptation to be fully animated using CGI technology, the first and so far only theatrical film adaptation to receive positive reviews, and the second Dr. Seuss film starring Jim Carrey after How the Grinch Stole Christmas (2000).