La clemenza di Tito (The Clemency of Titus), K. 621, is a two-act dramma per musica (now called opera seria) by Wolfgang Amadeus Mozart, composed to an existing Italian libretto by Pietro Metastasio as modified by Caterino Mazzolà. Mozart completed the work in the midst of composing Die Zauberflöte, his last opera. La clemenza di Tito premiered on 6 September 1791 at the Estates Theatre in Prague.

In 1791, the last year of his life, Mozart was already well advanced in writing Die Zauberflöte by July when he was asked to compose an opera seria. The commission came from the impresario Domenico Guardasoni, who lived in Prague and who had been charged by the Estates of Bohemia with providing a new work to celebrate the coronation of Leopold II, Holy Roman Emperor, as King of Bohemia. The coronation had been planned by the Estates in order to ratify a political agreement between Leopold and the nobility of Bohemia (it had rescinded efforts of Leopold's brother Joseph II to initiate a program to free the serfs of Bohemia and increase the tax burden of aristocratic landholders). Leopold desired to pacify the Bohemian nobility in order to forestall revolt and strengthen his empire in the face of political challenges engendered by the French Revolution. The ceremony was to take place on 6 September; Guardasoni had been approached about the opera in June. No opera of Mozart was more clearly pressed into the service of a political agenda than La clemenza di Tito, in this case to promote the reactionary political and social policies of an aristocratic elite. No evidence exists to evaluate Mozart's attitude toward this, or even whether he was aware of the internal political conflicts raging in the kingdom of Bohemia in 1791.

In a contract dated 8 July, Guardasoni promised that he would engage a castrato "of leading quality" (this seems to have mattered more than who wrote the opera); that he would "have the libretto caused to be written...and to be set to music by a distinguished maestro". The time was tight and Guardasoni had a get-out clause: if he failed to secure a new text, he would resort to La clemenza di Tito, a libretto written more than half a century earlier by Pietro Metastasio (1698–1782).

Metastasio's libretto had already been set by nearly 40 composers; the story is based on the life of Roman Emperor Titus, from some brief hints in The Lives of the Caesars by the Roman writer Suetonius, and was elaborated by Metastasio in 1734 for the Italian composer Antonio Caldara. Among later settings were Gluck's in 1752 and Josef Mysliveček's version in 1774. Mozart was definitely familiar with the libretto before composition; in 1770, he saw a production with his father of Michelangelo Valentini's setting in Cremona. There would be three further settings after 1791. Mozart was not Guardasoni's first choice. Instead, he approached Antonio Salieri, the most distinguished composer of Italian opera in Vienna and head of the music establishment at the imperial court. But Salieri was too busy, and he declined the commission, although he did attend the coronation.

The libretto was edited into a more useful state by the court poet Caterino Mazzolà. Unusually, in Mozart's personal catalogue of compositions, Mazzolà is credited for his revision with the note that the libretto had been "reworked into a true opera". Mazzolà conflated the original three act libretto into two acts, and none of the original Metastasio arias are from the original middle act. Mazzolà replaced a lot of the dialogue with ensembles and wrote a new act one finale, cobbled from lines in the original libretto, which presents the uprising, whereas Metastasio merely describes it.

Guardasoni's experience of Mozart's work on Don Giovanni convinced him that the younger composer was more than capable of working on the tightest deadline. Mozart readily accepted the commission given his fee would be twice the price of a similar opera commissioned in Vienna. Mozart's earliest biographer Niemetschek alleged that the opera was completed in just 18 days, and in such haste that the secco recitatives were supplied by another composer, probably Franz Xaver Süssmayr, believed to have been Mozart's pupil. Although no other documentation exists to confirm Süssmayr's participation, none of the secco recitatives are in Mozart's autograph, and it is known that Süssmayr traveled with Mozart to Prague a week before the premiere to help with rehearsals, proofreading, and copying. It has been suggested by scholars of Mozart that he had been working on the opera much longer, perhaps since 1789; however, all such theories have now been thoroughly refuted in the English-language musicological literature. The opera may not have been written in just 18 days, but it certainly ranks with Rossini's L'italiana in Algeri, Il barbiere di Siviglia and La Cenerentola as one of the operas written in the shortest amount of time that is still frequently performed today.

It is not known what Leopold thought of the opera written in his honor. Reports that his wife Maria Luisa of Spain dismissed it as una porcheria tedesca (literally in Italian "German swinishness", but most idiomatically translated "A German mess") do not pre-date 1871, in a collection of literary vignettes by Alfred Meissner about the history of Prague purportedly based on recollections of the author's grandfather, who was present for the coronation ceremonies.

It has been commonly asserted that Mozart himself was the conductor, although H. C. Robbins Landon says that is "most unlikely". The role of Sesto was taken by the castrato soprano Domenico Bedini. The opera was first performed publicly on 6 September 1791 at the Estates Theatre in Prague. While the orchestra was that of the theatre, the clarinet player Anton Stadler had journeyed to Prague with Mozart and played in the orchestra. It was for him that Mozart wrote two very prominent obbligati: for basset clarinet in Sesto's aria "Parto, parto, ma tu ben mio", and for basset horn in Vitellia's aria "Non più di fiori".