In music, the subdominant is the technical name for the fourth tonal degree of the diatonic scale. It is so called because it is the same distance "below" the tonic as the dominant is above the tonic - in other words, the tonic is the dominant of the subdominant. It also happens to be the note immediately "below" the dominant. In the C major scale (white keys on a piano, starting on C), the subdominant is the note F; and the subdominant chord uses the notes F, A, and C. In music theory, the subdominant chord is symbolized by the Roman numeral IV if it is within the major mode (because it is a major triad, for example F-A-C in C major) or iv if it is within the minor mode (because it is a minor triad, for example F-A♭-C in C minor).

Because ii₆, ii6
5, and neapolitan sixth chords contain the fourth scale degree in the bass, they are also considered subdominant harmonies because they substitute for the same harmonic purpose as chords built on the fourth scale degree.

A cadential subdominant chord followed by a tonic chord (the chord of the key of the piece) produces the so-called "plagal" (or "Amen") cadence.