Ionian mode, in Western music, the melodic mode with a pitch series corresponding to that of the major scale.

The Ionian mode was named and described by the Swiss humanist Henricus Glareanus in his music treatise Dodecachordon (1547). In that work Glareanus expanded the standing system of eight church modes—which had prevailed since the 9th century—to accommodate the increasingly common major and minor modes as well as the growing importance of harmony as a determinant of melodic motion. He added four new modes to the corpus: the Ionian, the Hypoionian, the Aeolian, and the Hypoaeolian. Both the Ionian mode and its plagal (lower-range) form, the Hypoionian mode, had C as their finalis (the tone on which a piece in a given mode ends). The Aeolian mode and its plagal counterpart, the Hypoaeolian mode, had their finalis on A. The pitch series of the Aeolian mode matches that of the natural minor scale.