Franz Krenn (26 February 1816 – 18 June 1897) was an Austrian composer and composition teacher.
Born in Droß, Krenn studied under Ignaz von Seyfried in Vienna. He served as organist in a number of Viennese churches and in 1862 became Kapellmeister of the Vienna Hofkirche. His compositions include masses, cantatas, oratorios, requiems, choral and solo songs, works for organ and piano, and symphonies.
From 1869 until 1893 Krenn taught harmony, counterpoint and composition at the Vienna Conservatory; during this time, he appears to have acquired the nickname 'Old Krenn', and is today often described as having been a rather pedantic teacher.
Among Krenn's well-known pupils are Gustav Mahler (who studied with him between 1875 and 1878), and Leoš Janáček. He also taught Mathilde Kralik, Hans Rott and Alexander von Zemlinsky.
Krenn died in Sankt Andrä-Wördern.