Sebastian z Felsztyna (– after 1543) was a Polish composer and music theorist, regarded as the greatest Polish composer of the early 16th century.
Sebastian z Felsztyna was probably born in Felsztyn, Kingdom of Poland (now Skelivka, Ukraine). In 1507 he entered Kraków University, the same year as his compatriot, the composer Mikołaj z Chrzanowa. While there he studied music and theology until 1509. He may have studied with the German composer Heinrich Finck while in Kraków. After graduating, he returned to Felsztyn, where he became a chaplain. He later went to Sanok, in southeastern Poland, where he was a provost.
Of Sebastian's musical compositions, three motets survive. all composed in 1522. They are for four voices, and use a plainchant tenor in long notes—an archaic practice at the time of publication—with the other voices sometimes engaging in imitation, in free counterpoint, or in more homophonic textures. All three are preserved a manuscript now held in Wawel Cathedral. While archaic in style, they show the influence of the Franco-Flemish school, and are a rare early example of four-voice polyphony in Poland.
Sebastian published a collection of his hymns in 1522 in Kraków, Latin: Aliquot hymni ecclesiastici, but no copies exist. His theoretical treatises cover the topics of notation and chant. His most popular was Latin: Opusculum musices, which was published in Kraków five years after theoretical treatises first appeared in Poland. It was most likely intended as an instructional work for singers. He produced Latin: Directiones musicae ad cathedralis ecclesiae Premisliensis usum (1544), and a version of St Augustine's Latin: De Musica, both of which proved less popular than the Latin: Opusculum, which was reprinted more than once. The relatively simple treatise lacked the normal discussion on counterpoint.
Sebastian's three motets all feature Gregorian melodies in the tenor in whole notes.