The ionian mode is a musical mode or diatonic scale. It was part of the music theory of ancient Greece, and was based around the relative natural scale in G (that is, the same as playing all the 'white notes' of a piano from G to G). This simple scale was called the hypophrygian mode in Greek theory, and the ionian mode must have been a different, perhaps chromatic, variation of this.

The term ionian mode fell into disuse in mediaeval Europe, as church music was based around eight musical modes: the relative natural scales in D, E, F and G, each with their authentic and plagal counterparts.

In 1547, Heinrich Glarean published his Dodecachordon. Central to its premise was the idea that there were twelve diatonic modes rather than eight. It seems that the additional modes were used in popular folk music, but were not part of the official church repetoire. Galrean borrowed the Greek term ionian for a quite different mode. He added ionian as the name of the new eleventh mode: the relative natural mode in C with the perfect fifth as its dominant, reciting note or tenor. The twelfth mode was the plagal version of the ionian mode, called hypionian (under ionian), based on the same relative scale, but with the major third as its tenor, and having a melodic range from a perfect fourth below the tonic, to a perfect fifth above it.

As mediaeval monophonic church music was replaced by polyphonic music, the folk modes added by Glarean became the basis of the minor/major division of classical European music: the ionian mode being the major mode.

The ionian mode of Glarean is effectively the same as the ancient Greek Lydian mode and the modern major mode.