Gustav Adolf Theill (7 February 1924 – 29 November 1997) was a German composer and musicologist.
His special love was the music of Johann Sebastian Bach.
He later dealt with Bach's works in his musicological writings, and the tonal language of his compositions was based on the harmonies of Felix Mendelssohn Bartholdy and Max Reger.
He attracted particular attention with the reconstruction of works by Johann Sebastian Bach that were lost or only preserved as fragments.
His most important work in this respect has become the reconstruction and completion of Bach's St Mark Passion.