James Patrick Donleavy, popularly known as J. P. Donleavy, (23 April 1926 – 11 September 2017) was an American-Irish author, short story writer, novelist, and playwright. Known for the dark humor in his writings, he first achieved critical acclaim with his picaresque novel The Ginger Man (1955), initially published in Paris. The novel became an international bestseller, selling 50 million copies worldwide. It is one of the best-selling books of all time and has been translated into over 30 languages. The novel is Donleavy's best-known work, and in 1998, it was ranked 99th by the Modern Library in its list of the "100 Best Novels of the 20th century".

Donleavy is also the author of A Fairy Tale of New York, published in 1973, and The Beastly Beatitudes of Balthazar B, published in 1968. He received the Bob Hughes Lifetime Achievement Award, funded by Bord Gáis Energy, for his contributions to Irish literature in 2015.

Donleavy was born in Brooklyn, to Irish immigrants Margaret and Patrick Donleavy, and grew up in the Bronx. His father was a firefighter, and his mother came from a wealthy background. He had a sister, Mary Rita, and a younger brother. He received his education at schools in the United States, then served in the US Navy during World War II. After the war ended, he moved to Ireland. In 1946 he began studying bacteriology at Trinity College Dublin, but left in 1949 before taking a degree.

Donleavy's first published work was a short story entitled A Party on Saturday Afternoon, which appeared in the Dublin literary periodical Envoy in 1950. He gained critical acclaim with his first novel, The Ginger Man (1955), which is one of the Modern Library 100 best novels. The novel, of which Donleavy's friend and fellow writer Brendan Behan was the first person to read the completed manuscript, was banned in Ireland and the United States by reason of obscenity.

The Ginger Man was known for its outspoken and comic lewdness. Lead character Sebastian Dangerfield was in part based on Trinity College companion Gainor Crist, an American Navy veteran also studying at Trinity College on the G.I. Bill, whom Donleavy once described in an interview as a "saint", though of a Rabelaisian kind.

Correctly or incorrectly, his initial works are sometimes grouped with the kitchen sink artists as well as the "Angry Young Men". Another novel, A Fairy Tale of New York, provided the title of the song "Fairytale of New York".

In March 2007, Donleavy was the castaway on BBC Radio 4's Desert Island Discs.

In 2015, Donleavy was the recipient of the Bob Hughes Lifetime Achievement Award at the Bord Gáis Energy Irish Book Awards.