William Sydney Porter (September 11, 1862 – June 5, 1910), known by his pen name O. Henry, was an American writer. O. Henry's short stories are known for their wit, wordplay, warm characterization, and clever twist endings.

Porter attended school for a short time, then clerked in an uncle's drugstore. At the age of 20 William Sydney Porter went to Texas, working first on a ranch and later as a bank teller. In 1887 he married and began to write freelance sketches. A few years later he founded a humorous weekly, the Rolling Stone. When this failed, he became a reporter and columnist on the Houston Post.

Released from prison in 1902, Porter went to New York City, his home and the setting of most of his fiction for the remainder of his life. Writing prodigiously under the pen name O. Henry, he completed one story a week for a newspaper, in addition to other stories for magazines. Popular collections of his stories included The Four Million (1906); Heart of the Westand The Trimmed Lamp (both 1907); The Gentle Grafter and The Voice of the City (both 1908); Options (1909); and Whirligigs and Strictly Business(both 1910).

O. Henry's most representative collection was probably The Four Million. The title and the stories answered the snobbish claim of socialite Ward McAllister that only 400 people in New York "were really worth noticing" by detailing events in the lives of everyday Manhattanites. In his most famous story, "The Gift of the Magi," a poverty-stricken New York couple secretly sell valued possessions to buy one another Christmas gifts. Ironically, the wife sells her hair so that she can buy her husband a watch chain, while he sells his watch so that he can buy her a pair of combs.

Incapable of integrating a book-length narrative, O. Henry was skilled in plotting short ones. He wrote in a dry, humorous style and, as in "The Gift of the Magi," frequently used coincidences and surprise endings to underline ironies. Even after O. Henry's death on June 5, 1910, stories continued to be collected: Sixes and Sevens (1911); Rolling Stones (1912); Waifs and Strays(1917); O. Henryana (1920); Letters to Lithopolis (1922); Postscripts (1923); and O. Henry Encore (1939).

Porter was a heavy drinker, and his health deteriorated markedly in 1908, which affected his writing.  He died on June 5, 1910, of cirrhosis of the liver, complications of diabetes, and an enlarged heart. After funeral services in New York City, he was buried in the Riverside Cemetery in Asheville, North Carolina.