Franz Xaver Süssmayr (1766 - September 17, 1803) was an Austrian composer.

Student and cantor in a benedictine convent (1779-87) in Kremsmünster , he became (after 1787) student of Mozart and A. Salieri in Vienna. In this city he also became the director and composer of Kärntnertortheater .

During Mozart's last days they discussed his Requiem and Süssmayr took on the task of completing the piece upon his death. Though criticised by some as not Mozartian, it is standard to play Süssmayr parts (for he composed the last fifth) in performances.

There was a rumor that he had an affair with Mozart's wife, Constanze.