Baron Jacques d'Adelswärd-Fersen (20 February 1880 – 5 November 1923) was a French novelist and poet. His life forms the basis of a fictionalised biography by Roger Peyrefitte.
In 1903 a scandal involving school pupils made him persona non grata in the salons of Paris, and dashed his marriage plans; after which he took up residence in Capri in self-imposed exile with his long-time lover, Nino Cesarini. He became a "character" on the island in the inter-war years, featuring in novels by Compton MacKenzie and others. His house, Villa Lysis, remains one of Capri's tourist attractions.
During his teenage years, the family spent long summer vacations at his grandfather's estate on Jersey, where Jacques reportedly had an intimate encounter with a blond Eton schoolboy.