Horrid Henry is a children's book series by Francesca Simon and illustrated by Tony Ross. It has been adapted for television, film and theatre.

Following the birth of her son Joshua in 1989, Francesca Simon was asked by Judith Elliott, an editor at Orion Books, to write a story about a horrid child. She also wanted to write about sibling rivalry and families where one child was considered "perfect" and the other "horrid". Inspiration also came from Cain and Abel, Oedipus Rex, Seinfeld, and Simon's own childhood, growing up in California the eldest of four siblings (including her sister Anne), desperately wanting to be an only child. They lived in small houses with confined spaces, with their father Mayo writing upstairs and telling them to be quiet. Simon also stated that she would be perfect and work hard at school, but be horrid and slam doors at home, describing herself as a "mixture of both Henry and Peter". Three more stories were written by Simon at Elliott's request to form a full book.

The first Horrid Henry book was written and published in 1994 by Orion. The books are a slice of life series featuring the titular Henry, a wildly misbehaved boy who will typically be faced with a problem and then will often retaliate in interesting ways that involve trickery, rule-breaking and elaborate practical jokes. Henry has a younger brother, Perfect Peter, who is the exact opposite. Henry despises Peter, whom he refers to as a "worm" or "toad". Almost every character is known by an alliterative nickname (Rude Ralph, Moody Margaret, Brainy Brian, etc.) with a few exceptions e.g. Mum, Dad and Miss Battle-Axe. Many of the books' stories were inspired by events from Simon's life and those of people that she knew, with additional ideas coming from her imagination. Each title in the main series consists of four short stories with black and white illustrations. The titles in the Early Readers series, aimed at younger readers, each consist of a single story with colour illustrations. Each story in that series, except Don't Be Horrid, Henry!, was originally published in a main series title. Don't Be Horrid, Henry! is an original story about Henry as a toddler, illustrated by Kevin McAleenan. Eleven fact books, written from Henry's point of view, have also been published along with eight joke books, eleven colour books and at least twelve activity books as well as a series of annuals. Up until 2015, 24 official Horrid Henry titles were published in the series. The series was published in 31 languages and has sold more than 21 million copies worldwide.

The Horrid Henry stories are read on audiobook by actress Miranda Richardson and published by Orion Audio. Released between 2000 and 2019, the audiobooks were produced by Peter Rinne and Nicholas Jones (for Strathmore Publishing), with music composed by Rinne and Dik Cadbury.

In April 2009, the US Sourcebooks' Jabberwocky imprint released four Horrid Henry paperbacks nationwide, which included Horrid Henry, Horrid Henry's Stinkbomb, Horrid Henry and the Mega-Mean Time Machine and Horrid Henry Tricks the Tooth Fairy. Sourcebooks has released additional Horrid Henry titles including activity pages, event kits and a teacher's guide. In 2019, a special one-off 25th Anniversary book, Horrid Henry: Up, Up and Away, was published.

On 23 June 2010, Vertigo Films announced that a live action 3-D film of Horrid Henry was in production. The film's plot focuses on Henry and The Purple Hand Gang fighting to prevent the closure of their school by an evil private school Headmaster. The film was released on 29 July 2011 in the United Kingdom and 11 January 2013 in Australia.