The Locrian mode is a musical mode or diatonic scale. It may be considered the major scale with a flatted or lowered second, third, fifth, sixth, and seventh. The Locrian mode can also be thought of as the major scale but starting on the seventh scale degree.

Some examples:

• The B Locrian mode is the C major scale starting on B. (B, C, D, E, F, G, A, B)
• The E Locrian mode is the F major scale starting on E. (E, F, G, A, Bb, C, D, E)

The Locrian mode can also be used to create a diminished chord, creating tension in music while still staying in key. In example one of the Locrian mode of C major, (B,D,F) create the diminished chord using notes 1,3,5 of the Locrian scale. It is called diminished because when compared to a B Major chord (B,#D,#F), we see that the D and F have been taken down one semitone. Since the #F which is number 5 of the chord has been reduced by a semitone, it is called diminished. This is the only major mode in which this occurs.

The Locrian mode was rarely used in classical music before the 1850's because of the large amount of dissonance created within the scale and its corresponding chord. In more recent musical pieces, the dissonance or musical inbalance created by the Locrian scale and chord have fallen back in favour (especially in Jazz) in order to create a sense of large tension.

The Locrian mode comes from the music theory of ancient Greece. However, what is now called the Locrian mode was what the Greeks called the Mixolydian mode. The original Greek Locrian mode seems to have been tuned to a natural A mode, but how it differed from the Aeolian and Hypodorian modes is unclear.