Horton Hatches the Egg is a children's book written and illustrated by Theodor Geisel under the pen name Dr. Seuss and published in 1940 by Random House. The book tells the story of Horton the Elephant, who is tricked into sitting on a bird's egg while its mother, Mayzie, takes a permanent vacation to Palm Beach. Horton endures a number of hardships but persists, often stating, "I meant what I said, and I said what I meant. An elephant's faithful, one hundred percent!" Ultimately, the egg hatches, revealing an elephant-bird, a creature with a blend of Mayzie's and Horton's features.

According to Geisel's biographers Judith and Neil Morgan, Geisel claimed the story was born in early 1940 when he left a window open in his studio, and the wind fortuitously blew a sketch of an elephant on top of a sketch of a tree. However, according to later biographer Charles Cohen, this account is probably apocryphal. He found elements of Horton in earlier Dr. Seuss works, most notably the 1938 short story "Matilda, the Elephant with a Mother Complex".

Horton Hatches the Egg was published to immediate critical acclaim and financial success and has remained popular with the general public. The book has also been used as the basis for academic articles on a variety of topics, including economics, Christianity, feminism, and adoption. Horton appeared again in the 1954 Dr. Seuss book Horton Hears a Who! These two books later provided the thrust of the plot for the 2000 Broadway musical Seussical.