Ronald William "Josh" Kirby (27 November 1928 – 23 October 2001) was a commercial artist born on the outskirts of Liverpool in the town of Waterloo, Lancashire, United Kingdom. With a career spanning across 6 decades he is world renowned as the original artist for Sir Terry Pratchett's Discworld novels, as well as some of the most critically acclaimed science fiction book cover illustrations.

From 1943 to 1949, Josh studied art techniques at the Liverpool City School of Art. His drawing course brought him the Intermediate certificate in Arts and Crafts and was followed by a painting course that led to his National Diploma in Design. It was here at the school that he picked up the nickname "Josh" which became his working name: 'When I was at Art School, some wag thought I painted like Sir Joshua Reynolds!'

When asked about influences, he most often names three past artists. First, Hieronymus Bosch, famous for those teeming, surreally fantastic landscapes of heaven and hell—including the Garden of Earthly Delights whose name was echoed in the Kirby collection In the Garden of Unearthly Delights. Next was Pieter Bruegel the Elder with his hauntingly detailed groups of warts-and-all Flemish peasants, not to mention the definitive portrayal of the colossal Tower of Babel which Kirby later spoofed in his movie poster for Monty Python's Life of Brian. Lastly was the muralist Frank Brangwyn (1867-1956), who made bold use of colour and monumental compositions on a large scale. Of all three Kirby said, 'I try to become more like these whilst contributing my personal viewpoint.'

After an early commission by Liverpool City Council to paint their Mayor, Kirby decided against the staid life of portrait-painting which had become a possibility while in art school. His first published painting was produced in 1954 for the cover of Cee-Tee Man, a now largely forgotten science fiction novel by Dan Morgan. In 1956 he touched the beginning of the blockbuster James Bond phenomenon with a cover for the first Pan paperback edition of Ian Fleming's Moonraker.

His cover illustration career flourished into the 1970s when Kirby began freelancing for the film publicity agency FEREF, working with the likes of the designer Eddie Paul on a number of posters. In 1979 he created an illustration for Monty Python's Life of Brian but it was unused, apparently because several of the character illustrations were considered too risqué for cinema-goers. He also lent his talent to depict the characters in Return of the Jedi. When the market for poster illustration dried up towards the middle of the 1980s, Kirby began what would prove to be a long relationship working on the book covers for author Terry Pratchett’s Discworld series of novels. Kirby created over 400 cover paintings, including the literary works of Ian FlemingRay BradburyIsaac AsimovAlfred HitchcockJimmy SangsterRichard MathesonJack KerouacJules VerneEdgar Rice Burroughs and H. G. Wells.

In 1991, Paper Tiger Books published a graphic album collecting some commercial and private works by Kirby, titled In the Garden of Unearthly Delights (a reference to Hieronymus Bosch's painting The Garden of Earthly Delights). This was followed in 1999 by another graphic album titled A Cosmic Cornucopia, which includes extensive text by David Langford and two chapters dedicated to his work for Terry Pratchett's Discworld series.

Kirby worked almost exclusively in oils.

Kirby died of natural causes in his sleep at home in Shelfanger near Diss in Norfolk at the age of 72.