Egon Joseph Wellesz (21 October 1885 – 9 November 1974) was an Austrian-born British composer, teacher and musicologist, notable particularly in the field of Byzantine music.

Although both parents of Wellesz's were Hungarian Christians, they both had Jewish ancestry. Even though he had Jewish ancestry and had a Protestant upbringing, he still later converted to Catholicism. Wellesz studied in Vienna under Arnold Schoenberg — purportedly his first private pupil — as well as Guido Adler, who founded the musicological institute in Vienna and was a leading editor of the Austrian Denkmaler. These dual influences shaped much of his musical and scholarly thought. In 1913, Wellesz embarked upon what would become a lifelong interest in the musical achievements of Byzantium.

Wellesz left Austria for England in the wake of the Anschluss — more specifically, Wellesz was in Amsterdam at the time by good fortune, to hear his orchestral piece Prosperos Beschwörungen conducted by Bruno Walter on that day.[1] Altogether he wrote nine symphonies and an equal number of string quartets, the former starting, in 1945,[2] only with his arrival in England and the latter series of works spread throughout his life. Other compositions by him include operas, one of which (Die Bakchantinnen) was revived and recorded a few years ago; an octet with the same instrumentation as Schubert's; piano and violin concertos (one of each); and a suite for violin and orchestra. Stylistically his earliest music, somewhat like that of Ernst Krenek, is in a harsh but recognisably tonal style; there is a definite second period of sorts around the time of the first two symphonies (1940s) in which his music has a somewhat Brucknerian sound — in the symphonies sometimes an equal breadth,[2] though still with something of a 20th-century feel and harmonies — but after his fourth symphony (the Austriaca) his music is more tonally vague in character, with serial techniques used. This idiom is consistent with hints of tonality, such as can be found in his eighth string quartet.

Despite his composing, Wellesz remains best known for his extensive scholarly contributions to the study of Byzantine music. These contributions brought for him an honorary doctorate from Oxford (where he later taught) in 1932.