Karl Gustav Fellerer (7 July 1902 – 7 January 1984) was a German musicologist.
He wrote monographs on Palestrina, Handel, Mozart, and Max Bruch and was editor of several musical journals.
He studied in Regensburg, then in Munich, where Joseph Haas was among his teachers, and in Berlin where in 1925 he gained a Ph.D.
Aged 81, he died on 7 January 1984 in München Hauptbahnhof when he returned from a meeting on church music in Innsbruck.
Fellerer was member of several academies of science, including those in London, Brussels, and Copenhagen.