Louise Perkins Fitzhugh (October 5, 1928 – November 19, 1974) was an American writer and illustrator of children's books, best known for her 1964 novel Harriet the Spy. This 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, 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.

Fitzhugh died at age 46 from a brain aneurysm.

Born in Memphis, Tennessee, she attended Miss Hutchison's School and three different universities, without obtaining a degree. According to her obituary in the New York Times, Fitzhugh graduated from Barnard College in 1950. She lived most of her adult life in New York City and had houses in both Long Island and Bridgewater, Connecticut. She was married briefly to Ed Thompson, whom she dated in high school. After high school, she primarily dated women. Fitzhugh was the illustrator of the 1961 children's book Suzuki Beane, a parody of Eloise; while Eloise lived in the Plaza, Suzuki was the daughter of beatnik parents and slept on a mattress on the floor of a Bleecker Street pad in Greenwich Village. Fitzhugh worked closely with author Sandra S.