La clemenza di Tito, K. 621, is an opera seria in two acts composed by Wolfgang Amadeus Mozart to an Italian libretto by Caterino Mazzolà, after Pietro Metastasio. Mozart began the work while he was finishing Die Zauberflöte, the last of his principal operas. La clemenza di Tito premiered on 6 September 1791 at the Estates Theatre in Prague.

The Köchel catalogue is a chronological catalogue of compositions by Wolfgang Amadeus Mozart, originally created by Ludwig Ritter von Köchel, in which the entries are abbreviated K. or KV. Its numbers reflect the ongoing task of compiling the chronology of Mozart's works, and provide a shorthand reference to the compositions.