Joseph Ferdinand Sonnleithner (3 March 1766 – 25 December 1835) was an Austrian librettist, theater director archivist and lawyer. He was the son of Christoph Sonnleithner, brother of Ignaz von Sonnleithner and uncle of Franz Grillparzer and Leopold von Sonnleithner. He was a personal friend and attorney of Ludwig van Beethoven, and he wrote numerous librettos, among them, Beethoven's stage opera Fidelio, Faniska by Luigi Cherubini and Agnes Sorel by Adalbert Gyrowetz.