L'Arianna (SV 291, Ariadne) is the lost second opera by Italian composer Claudio Monteverdi. One of the earliest operas in general, it was composed in 1607–1608 and first performed on 28 May 1608, as part of the musical festivities for a royal wedding at the court of Duke Vincenzo Gonzaga in Mantua. All the music is lost apart from the extended recitative known as "Lamento d'Arianna" ("Ariadne's Lament"). The libretto, which survives complete, was written in eight scenes by Ottavio Rinuccini, who used Ovid's Heroides and other classical sources to relate the story of Ariadne's abandonment by Theseus on the island of Naxos and her subsequent elevation as bride to the god Bacchus.

The opera was composed under severe pressure of time; the composer later said that the effort of creating it almost killed him. The initial performance, produced with lavish and innovative special effects, was highly praised, and the work was equally well received in Venice when it was revived under the composer's direction in 1640 as the inaugural work for the Teatro San Moisè.

Rinuccini's libretto is available in a number of editions. The music of the "Lamento" survives because it was published by Monteverdi, in several different versions, independently from the opera. This fragment became a highly influential musical work and was widely imitated; the "expressive lament" became an integral feature of Italian opera for much of the 17th century. In recent years the "Lamento" has become popular as a concert and recital piece and has been frequently recorded.