The Hypophrygian mode, literally meaning 'below Phrygian', is a musical mode or diatonic scale of ancient Greece that was based upon the Phrygian tetrachord: a series of rising intervals of a whole tone, followed by a semitone, followed by another whole tone. The rising scale for the octave is a single tone followed by two conjoint Phrygian tetrachords. This is the same as playing all the white notes of a piano from G to G: G | A B C D | (D) E F G. Confusingly, this scale in mediaeval and modern music theory came to be known as the Mixolydian mode.

The mediaeval music scholars, misunderstanding the Latin texts by Boethius of how the Greek modes were reckoned, used the term Hypophrygian to describe the fourth mode of church music. This mode is the plagal counterpart of the authentic third mode, which was dubbed Phrygian. The ecclesiastical Hypophrygian mode is based on the relative scale of 'white notes' from E to E, with the musical dominant, the reciting note , or tenor at the perfect fourth on the scale (or A, in the E to E scale). The melodic range of the ecclesiatical Hypophrygian mode ranges from the perfect fourth below the tonic to the perfect fifth above.