Tristan und Isolde, WWV 90, is an opera in three acts by Richard Wagner to a German libretto by the composer, based largely on the 12th-century romance Tristan and Iseult by Gottfried von Strassburg. It was composed between 1857 and 1859 and premiered at the Königliches Hoftheater und Nationaltheater in Munich on 10 June 1865 with Hans von Bülow conducting. Wagner referred to the work not as an opera, but called it "eine Handlung".

Gottfried von Strassburg is the author of the Middle High German courtly romance Tristan, an adaptation of the 12th-century Tristan and Iseult legend. Gottfried's work is regarded, alongside the Nibelungenlied and Wolfram von Eschenbach's Parzival, as one of the great narrative masterpieces of the German Middle Ages. He is probably also the composer of a small number of surviving lyrics. His work became a source of inspiration for Richard Wagner's opera Tristan und Isolde (1865).