In music or music theory, the supertonic is the second degree of the scale, it is the second note of a diatonic scale. For example, in the C major scale (white keys on a piano), the supertonic is the note D; and the supertonic chord uses the notes D, F, and A. In music theory, the supertonic chord is symbolized with the Roman numeral II if major or ii if minor.

The super tonic is the root of the minor seventh chord and the fifth of the dominant seventh chord.

"Supertonic" also refers to a relationship of musical keys. For example, relative to the key of C major, the key of D major is the mediant. Modulation (changes of key) to the mediant are relatively rare, compared with, say, modulation into the dominant.

In German theory derived from Hugo Riemann the supertonic is considered the subdominant parallel.