In music from Western culture, a diminished octave () is an interval produced by narrowing a perfect octave by a chromatic semitone. As such, the two notes are denoted by the same letter but have different accidentals. For instance, the interval from C<sub>4</sub> to C<sub>5</sub> is a perfect octave, twelve semitones wide, and both the intervals from C<sub>4</sub> to C<sub>5</sub> and from C<sub>4</sub> to C<sub>5</sub> are diminished octaves, spanning eleven semitones.
Being diminished, it is considered a dissonant interval. The diminished octave is enharmonically equivalent to the major seventh.