Mikko Kyösti Heiniö (born 18 May 1948) is a Finnish composer and musicologist.
Heiniö was born in in Tampere, and studied composition with Joonas Kokkonen and piano with Liisa Pohjola at the Sibelius Academy in Helsinki from 1971 to 1975, and then studied composition with Witold Szalonek in West Berlin from 1975–77 while at the same time beginning studies in musicology at the University of Helsinki. He earned a diploma in composition from the Sibelius Academy in 1977, and a doctorate in musicology in 1984 from the University of Helsinki, where he lectured between 1977 and 1985 (Murtomäki 2001). He was appointed professor of musicology at the University of Turku in 1986, composer-in-residence of the Turku Philharmonic Orchestra in 1997, and has served as Chairman of the Society of Finnish Composers since 1992.
Heiniö's compositions include seven piano concertos and numerous symphonic and chamber works. He has written three operas: the church opera Riddaren och draken (The Knight and the Dragon, 2000), Käärmeen hetki (The Hour of the Serpent), premièred in September 2006, and Eerik XIV (Erik XIV), premièred in November 2011.
Heiniö is also a noted writer on music, specializing in the subject of new Finnish music. He has written thirty books and nearly a hundred articles. His music has been recorded on the BIS label, among others.