En música un acorde de séptima, es un acorde formado por la superposición de cuatro notas musicales, cada una distante una tercera respecto de la nota precedente. De donde resulta que las tres notas sucesivas a la nota fundamental se encuentran a intervalos respectivos de una tercera, una quinta y una séptima de la nota base. Un acorde de séptima se puede interpretar como formado mediante una tríada a la que se agrega una nota que forma un intervalo musical de una séptima con respecto a la raíz del acorde. La teoría clásica de la música considera disonantes a todos los acordes de séptima, ya que el intervalo de séptima que contienen es considerado también un intervalo disonante.

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A seventh chord is a chord consisting of a triad plus a note forming an interval of a seventh above the chord's root. When not otherwise specified, a "seventh chord" usually means a dominant seventh chord: a major triad together with a minor seventh. However, a variety of sevenths may be added to a variety of triads, resulting in many different types of seventh chords. In its earliest usage, the seventh was introduced solely as an embellishing or nonchord tone. The seventh destabilized the triad, and allowed the composer to emphasize movement in a given direction. As time passed and the collective ear of the western world became more accustomed to dissonance, the seventh was allowed to become a part of the chord itself, and in some modern music, jazz in particular, nearly every chord is a seventh chord. Additionally, the general acceptance of equal temperament during the 19th century reduced the dissonance of some earlier forms of sevenths.

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