Orlando furioso (Italian pronunciation: [orˈlando fuˈrjoːzo, -so]; The Frenzy of Orlando) is an Italian epic poem by Ludovico Ariosto which has exerted a wide influence on later culture. The earliest version appeared in 1516, although the poem was not published in its complete form until 1532. Orlando furioso is a continuation of Matteo Maria Boiardo's unfinished romance Orlando innamorato (Orlando in Love, published posthumously in 1495). In its historical setting and characters, it shares some features with the Old French La Chanson de Roland of the eleventh century, which tells of the death of Roland. The story is also a chivalric romance which stemmed from a tradition beginning in the Late Middle Ages and continuing in popularity in the 16th century and well into the 17th.

Orlando is the Christian knight known in French and English as Roland. The story takes place against the background of the war between Charlemagne's Christian paladins and the Saracen army that has invaded Europe and is attempting to overthrow the Christian empire. The poem is about knights and ladies, war and love, and the romantic ideal of chivalry. It mixes realism and fantasy, humor and tragedy. The stage is the entire world, plus a trip to the Moon. The large cast of characters features Christians and Saracens, soldiers and sorcerers, and fantastic creatures including a gigantic sea monster called the Orc and a flying horse called the hippogriff. Many themes are interwoven in its complicated episodic structure, but the most important are the paladin Orlando's unrequited love for the pagan princess Angelica, which drives him mad; the love between the female Christian warrior Bradamante and the Saracen Ruggiero, who are supposed to be the ancestors of Ariosto's patrons, the House of Este of Ferrara; and the war between Christian and Infidel.

The poem is divided into forty-six cantos which contain a varying number of eight-line stanzas in ottava rima (a rhyme scheme of abababcc). Ottava rima had been used in previous Italian romantic epics, including Luigi Pulci's Morgante and Boiardo's Orlando Innamorato. Ariosto's work is 38,736 lines long in total, making it one of the longest poems in European literature.

Ariosto began working on the poem around 1506, when he was 32. The first edition of the poem, in 40 cantos, was published in Ferrara in April 1516 and dedicated to the poet's patron Ippolito d'Este. A second edition appeared in 1521 with minor revisions.

Ariosto continued to write more material for the poem and in the 1520s he produced five more cantos, marking a further development of his poetry, which he decided not to include in the final edition. They were published after his death by his illegitimate son Virginio under the title Cinque canti and are highly regarded by some modern critics. The third and final version of Orlando Furioso, containing 46 cantos, appeared in 1532.

Ariosto had sought stylistic advice from the humanist Pietro Bembo to give his verse the last degree of polish and this is the version known to posterity.

The first English translation by John Harington was published in 1591 at the behest of Queen Elizabeth I, who reportedly banned Harington from court until the translation was complete.

Ariosto's poem is a sequel to Matteo Maria Boiardo's Orlando Innamorato (Orlando in Love). One of Boiardo's main achievements was his fusion of the Matter of France (the tradition of stories about Charlemagne and paladins such as Roland) with the Matter of Britain (the legends about King Arthur and his knights). The latter contained the magical elements and love interest that were generally lacking in the more austere and warlike poems about Carolingian heroes.