Franz Xaver Süßmayr (also spelled Süssmayr, born Schwanenstadt, Austria, 1766; died Vienna, 17 September 1803) was an Austrian composer. He is often remembered for finishing Mozart’s Requiem which Mozart had not finished when he died.
His father was a sacristan and teacher. His mother died when he was 6, and he left home at 13. He went to Kremsmünster to study in a Benedictine monastery. When his voice changed, he became a member of the orchestra as a violinist.
The abbey performed operas and Singspiele, so he had the opportunity to study the operas of Christoph Willibald Gluck and Antonio Salieri. He composed a number of stage works and a good deal of church music for the abbey.
He helped Wolfgang Amadeus Mozart by copying out the music for La clemenza di Tito and Die Zauberflöte in 1791. In the same year Mozart died. He had not finished composing his Requiem. Süssmayr finished composing it and handed the completed work to Constanze, Mozart’s widow.
He composed Singspiele (German operas with spoken dialogue), and became Kapellmeister at the National Theater From this time until 1800, he was very successful and popular on the Viennese music scene. However, he became ill with tuberculosis and had to stop work. He died three years later in 1803.