Almira, Königin von Castilien ("Almira, Queen of Castile", HWV 1; full title: Der in Krohnen erlangte Glücks-Wechsel, oder: Almira, Königin von Castilien) is Handel's first opera, composed when he was 19 years old. It was first performed in Hamburg in January 1705.

Georg Friedrich Händel came to the city of Hamburg in the summer of 1703 and played as a violinist in the theatre at the Gänsemarkt, the local market place. On later occasions, he also played the harpsichord in the orchestra. His first opera – announced as a Singspiel although it has no spoken dialogue – was premiered on 8 January 1705, after being composed in the months directly preceding this.

An Italian libretto was written by Giulio Pancheri in Venice in 1691 for Giuseppe Benevento's opera L'Almira. The German translation used by Handel was made by Friedrich Christian Feustking. The recitatives of the opera are in German, and while most of the arias are also in German, many are in Italian, as was the custom at the opera house in Hamburg.

Almira is the sole example among Handel's many operas with no role for a castrato.

Almira was a resounding success. The opera was performed twenty times in total until its place was taken by Handel's next opera, Nero, the music of which has not been preserved.

The first modern performance of Almira took place on 23 February 1985, Handel's 300th birthday, at Leipzig's Städtische Oper.

As with all of Handel's operas, after going unperformed for many years, Almira is presented by opera houses and festivals today. Among other performances, Almira was staged by operamission in New York City in 2012 and by the Boston Early Music Festival in 2013.

The fictional story is set in medieval Valladolid.