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

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