For mediant in mathematics, see mediant (mathematics).

In music, the mediant is the third degree of the diatonic scale, being the "middle" note of the tonic triad. For example, in the C major scale (white keys on a piano, starting on C), the mediant is the note E; and the mediant chord uses the notes E, G, and B. In music theory, the mediant chord is symbolized by the Roman numeral III.

"Mediant" also refers to a relationship of musical keys. For example, relative to the key of C minor, the key of E flat major is the mediant, and often serves as a mid-way point between I and V (hence the name). Tonicization or modulation to the mediant is quite common in pieces written in the minor mode, and usually serves as the second theme group in sonata forms, since it is very easy to tonicize III in minor (no need to alternate notes). Tonicization of III in major is quite rare in classical harmony, compared with, say, modulation to the V in major, but mediant tonicization in major is an important feature of late romantic music.

In German theory derived from Hugo Riemann the mediant in major is considered the dominant parallel, Dp, and in minor the tonic parallel, tP.