Johann Nikolaus Harnoncourt (6 December 1929 – 5 March 2016) was an Austrian conductor, known for his historically informed performances. He specialized in music of the Baroque period, but later extended his repertoire to include Classical and early Romantic works. Among his best known recordings are those of Bach, whose 193 cantatas he recorded with Gustav Leonhardt.

Starting out as a classical cellist, he founded his own period instrument ensemble, Concentus Musicus Wien, in 1953, and became a pioneer of the Early Music movement. Around 1970, Harnoncourt began conducting opera and concert performances, soon leading international symphony orchestras, and appearing at leading concert halls, operatic venues and festivals. In 2001 and 2003, he conducted the Vienna New Year's Concert. Harnoncourt was also the author of several books, mostly on subjects of performance history and musical aesthetics.