William Forsell Kirby (14 January 1844 – 20 November 1912) was an English entomologist and folklorist.
He was born in Leicester. He was the eldest son of Samuel Kirby, who was a banker. He was educated privately, and became interested in butterflies and moths at an early age. The family moved to Brighton, where he became acquainted with Henry Cooke, Frederick Merrifield and J. N. Winter. He published the Manual of European Butterflies in 1862.
In 1867 he became a curator in the Museum of the Royal Dublin Society, and produced a Synonymic Catalogue of Diurnal Lepidoptera (1871; Supplement 1877).
In 1879 Kirby joined the staff of the British Museum (Natural History) as an assistant, after the death of Frederick Smith. He published a number of catalogues, as well as Rhopalocera Exotica (1887–1897) and an Elementary Text-book of Entomology. He retired in 1909.
Kirby had a wide range of interests, knew many languages and fully translated Finland's national epic, the Kalevala, from Finnish into English. Kirby's translation, which carefully reproduces the Kalevala meter, was a major influence on the writings of J.R.R. Tolkien, who first read it in his teens.
Kirby provided many footnotes to Sir Richard Burton's translation of the Arabian Nights.
Kirby also did important work on orthopteroid insects including a three volume Catalogue of all known species (1904, 1906, 1910). A short biography of Kirby, with particular reference to his work on phasmids was published by Bragg in 2007.