Harriet the Spy is a children's novel written and illustrated by Louise Fitzhugh that was published in 1964. It has been called "a milestone in children's literature" and a "classic". In the U.S., it ranked number 12 in the 50 Best Books for Kids and number 17 in the Top 100 Children's Novels on two lists generated in 2012.

Louise Perkins Fitzhugh was an American writer and illustrator of children's books. Fitzhugh is best known for her 1964 novel Harriet the Spy, a fiction work about an adolescent girl's predisposition with a journal covering the foibles of her friends, her classmates, and the strangers she is captivated by. The novel was later adapted into a live action film in 1996. The sequel novel, The Long Secret, was published in 1965, and its follow-up book, Sport, was published posthumously in 1979. Fitzhugh also wrote Nobody's Family Is Going to Change, which was later adapted into a short film and a play.