In music or music theory a ninth is the note nine scale degrees from the root of chord and also the interval between the root and the ninth.

A ninth is a compound second, but the ninth is considered less dissonant than the second.

A ninth chord is a chord with a ninth, and is an extended chord. Added tone chords with what might be called an added second are usually said to have an added ninth, two fifths up from the root.

"The Ninth" commonly refers to Beethoven's Ninth Symphony.