William Wymark Jacobs (8 September 1863 – 1 September 1943) was an English author of short fiction and drama. He is best known for his story "The Monkey's Paw".

He was born in 1863 at 5, Crombie's Row, Mile End Old Town (not Wapping, as is often stated), London, to William Gage Jacobs, wharf manager, and his wife Sophia. His father managed the South Devon wharf in Lower East Smithfield, by the St Katherine Docks and, according to the Oxford Dictionary of National Biography, "the young Jacobs spent much time on Thames-side, growing familiar with the life of the neighbourhood" and "ran wild in Wapping". Jacobs and his siblings were still young when their mother died. Their father then married his housekeeper and had seven more children, including illustrator Helen Jacobs. Jacobs attended a private London school before Birkbeck College (Birkbeck Literary and Scientific Institution, now part of the University of London), where he befriended William Pett Ridgcap.

In 1879, Jacobs began work as a clerk in the Post Office Savings Bank. By 1885 he had his first short story published, but success came slowly. Yet Arnold Bennett in 1898 was astonished to hear that Jacobs had turned down £50 for six short stories. He was financially secure enough to be able to leave the Post Office in 1899.

Jacobs is remembered for a macabre tale, "The Monkey's Paw", (published 1902 in a short-story collection, The Lady of the Barge) and several other ghost stories, including "The Toll House" (from the 1909 collection Sailors' Knots) and "Jerry Bundler" (from the 1901 Light Freights). Most of his work was humorous. His favourite subject was marine life – "men who go down to the sea in ships of moderate tonnage", said Punch, reviewing his first collection, Many Cargoes, which gained popular success on publication in 1896.

Michael Sadleir has said of Jacobs's fiction, "He wrote stories of three kinds: describing the misadventures of sailor-men ashore; celebrating the artful dodger of a slow-witted village; and tales of the macabre."

Many Cargoes was followed by the novel The Skipper's Wooing in 1897, and another collection of short stories, Sea Urchins (1898), confirmed his popularity. Other titles included Captains All, Sailors' Knots, and Night Watches. The title of the last reflects the popularity of an enduring character: the night-watchman on the wharf in Wapping, recounting the preposterous adventures of his acquaintances Ginger Dick, Sam Small, and Peter Russet. These three characters, pockets full after a long voyage, took lodgings together, set on enjoying a long spell ashore, but the crafty inhabitants of dockland London soon relieved them of their funds, assisted by their own fecklessness and credulity. Jacobs showed a delicacy of touch in his use of the coarse vernacular of the East End of London, which attracted the respect of P. G. Wodehouse, who mentions Jacobs in his autobiographical work Bring on the Girls!, written with Guy Bolton and published in 1954.

The stories in Many Cargoes had varied previous serial publication, while those in Sea Urchins were for the most part published in Jerome K. Jerome's Idler. From October 1898, Jacobs's stories appeared in The Strand, which provided him with financial security almost up to his death.

John Drinkwater described Jacobs's fiction as "in the Dickens tradition".