Matthaiou began playing professional basketball, as well as many other sports, with the Greek club
Aris Thessaloniki in 1945. His father was
Manthos Matthaiou, who was the
President of the
Aris Thessaloniki Sports Club, who was killed in 1941, during the Italian air bombings of
Thessaloniki, during the
Greco-Italian War. In 1949, Matthaiou transferred to the Greek club
Panathinaikos Athens. He also played with the Greek club
Panionios Athens, the
Italian League club
Varese, and the Greek club
Sporting Athens. With Panathinaikos, Matthaiou won 3
Greek League championships, in the years 1950, 1951, and 1954. He also played at the International Club Cup Tournament (the forerunner of the
EuroLeague) in 1955, at the Viareggio Tournament, where he was the leading scorer and
MVP. ==National basketball team career==