Karl V. is an opera, described as a Bühnenwerk mit Musik (stage work with music) by Ernst Krenek, his opus 73.

[1] The first completed full-length twelve-tone opera[2] tells the story of Emperor Charles V's life in a series of flashbacks on a split stage, devices which the composer only much later recognized as "cinematic" in style;[3] there is also some use of Sprechstimme.

Originally commissioned in 1930 by the Vienna State Opera for performance in 1934, this much anticipated work[4] became a cause célèbre when the production was cancelled after Krenek was blacklisted in Germany by the Nazi government immediately following the German parliamentary elections in March 1933.

[5] The composer believed it was its strong emphasis on Christian universality that made Karl V. "utterly intolerable" to the Nazis.

A fully staged production of the opera was performed in the Festspielhaus of the Bregenzer Festspiele in the summer of 2008, and is available on DVD.