The Mayor finds out from Dr. LaRue (Isla Fisher) that Whoville will be destroyed if Horton does not find a "safer, more stable home." Horton resolves to place the speck atop Mt. Nool, the safest place in the jungle. The Sour Kangaroo (Carol Burnett), not believing Horton's beliefs, tries to get Horton to give up the speck, but Horton refuses. Also taking force toward Horton are the Wickersham Brothers, a group of bullying monkeys who love making misery. The Kangaroo eventually enlists a vulture named Vlad Vladikoff (Will Arnett) to get rid of the speck by force.

Vlad manages to steal the clover from Horton and drops it into a field of identical clovers. After picking nearly 3,000,000 clovers, Horton eventually recovers the clover. The Kangaroo finds out that Horton still has the speck, and decides to rally the jungle community into confronting Horton by telling lies that Horton's goal will lead to anarchy. The Kangaroo demands Horton to give up the speck. When Horton refuses, in spite of his heartfelt speech, she orders the animals to rope and cage him and to have Whoville destroyed by means of a pot of boiling beezelnut oil as punishment for "making up stories about people on specks". The Mayor enlists all of his people to make noise by shouting "We are here", as well as playing a variety of instruments, so the animals may hear them. Jane takes the clover and drops it into the boiling oil. At the last minute, JoJo grabs the horn used to project Horton's voice, runs up the highest tower, and yells "YOPP!", breaking through the sound barrier just before the speck hits the oil. Kangaroo's son, Rudy (Josh Flitter) grabs the clover and returns it to Horton, refusing his mother's orders to return to her pouch. The animals realize that Horton was right and, thinking that she was just trying to crush Horton's spirit and build her own reputation in the jungle, shun the Kangaroo for tricking them. While being praised for his integrity by his neighbors, Horton forgives the Kangaroo, and she accepts his friendship, finally believing that there are people on the speck. Here, the people of Whoville and the animals of Nool gather in song and recite the chorus from "Can't Fight This Feeling". The film ends with the narrator revealing that the Jungle of Nool, or rather Earth, is just one speck, among numerous others, floating in the universe.

Critical reception
In contrast to the mixed reaction to the other big screen adaptations of Seuss's work, the film received generally positive reviews from film critics. Review aggregator Rotten Tomatoes reports that 79% of critics gave the film positive reviews, based on 131 reviews, classifying the film as "Certified Fresh". The consensus statement reads, "Horton Hears A Who! is both whimsical and heartwarming, and is the rare Dr. Seuss adaptation that stays true to the spirit of the source material." Another review aggregator, Metacritic, calculated a score of 71 out of 100 based on 31 reviews, indicating "generally favorable reviews."

In contrast, Brian Eggert of Deep Focus gave it one and a half stars out of four, criticizing its numerous pop-culture references, calling it a "mish-mash of incoherent babble" and claiming it ends up "reducing Seuss' otherwise admirable message to ordinary storytelling, when Seuss' work is anything but."

Box office
Horton Hears a Who! was a box office success, grossing a total of $297,138,014 on a $85 million budget. $154,529,439 came from the United States and Canada, and $142,608,575 from other territories.

In its opening weekend, the film grossed $45,012,998 in 3,954 theaters, averaging $11,384 per theater in the United States and Canada, and ranking #1 at the box office. The film previously had the 4th largest opening weekend in March, behind Ice Age, Ice Age: The Meltdown and 300, and as of September 2012, it ranks on the 9th place. In the United States and Canada, Horton Hears a Who was also the #1 film its second weekend of release, grossing $24,590,596 over the Easter frame, in 3,961 theaters and averaging $6,208 per venue. It dropped to #2 in its third weekend grossing $17,740,106 in 3,826 theaters and averaging $4,637 per venue. Its fourth weekend ranked at #4 grossing $9,115,987 in 3,571 theaters and averaging $2,553 per venue. Its fifth weekend ranked at #6, grossing $5,920,566 in 3,209 theaters and averaging $1,845 per venue.