Greek classical composer (b. 1910, Kavala - d. 1989, Athens).

He studied piano with Marika Laspopoulou and composition with Alekos Kontis at the Hellenic Conservatory in Athens (1922–34), as well as the piano and orchestration with Emilios Riadis in Thessaloniki (1928–9). In 1949, he visited the major European music centres on a UNESCOgrant and he became familiar with new compositional developments. In particular, in Paris he attended Arthur Honegger's class. Between 1951 and 1961 he taught music at an Athens high-School and from 1953 he was professor of counterpoint and composition at the Hellenic Conservatory. He was the first president of both the Greek section of the ISCM (1964–75) and the Hellenic Association for Contemporary Music (1965–75).

Papaioannou was the first musician to systematically teach atonal, 12-note and serial techniques before 1970. His students include Michael Adamis, Theodore Antoniou, Georges Aperghis, Argyris Kounadis (Αργύρης Κουνάδης) and Dimitri Terzakis. His compositional career falls into five main phases: Early Impressionist (1932–8), National School (1939–43), Hindemithian neo-classicism combined with elements from Byzantine music (1944–1952), 12-note period (1953–1966) and the last period (1966–1989) in which he developed an entirely personal technique.

Since 2000, there is a Festival honoured to him (Φεστιβάλ Παπαϊωάννου - Festival Papaioannou) on his town of birth, Kavala.