In music, the dominant 7♯9 chord ("dominant seven sharp nine" or "dominant seven sharp ninth") is a chord built by combining a dominant seventh, which includes a major third above the root, with an augmented second, which is the same pitch, albeit given a different note name, as the minor third degree above the root. This chord is used in many forms of contemporary popular music, including jazz, funk, R&B, rock and pop. As a dominant chord in diatonic harmony, it most commonly functions as a turnaround chord, returning to the tonic.

The chord is also sometimes colloquially known, among pop and rock guitarists, as the "Hendrix chord" or "Purple Haze chord", nicknamed for guitarist Jimi Hendrix, who showed a preference for the chord and did a great deal to popularize its use in mainstream rock music. When used by The Beatles it has been called the "Gretty chord" although this can refer to a distinct six-string version.

There are two main ways to spell the chord, depending on the musical style, kind of musical notation (score or chord symbols), and personal taste. One consists of a dominant seventh chord with an added minor third placed one or more octaves over the major third (a minor tenth); the other, more common, consists of a dominant seventh chord with an added augmented ninth. The former can be written in popular chord symbol notation systems as 7♭10 and the latter as 7♯9. 7b10 is unorthodox way to write but it is in use on some contexts.

Sometimes, in a publication that includes both traditionally notated music and chord symbols, the music is notated with both a natural and a flattened third, while the chord symbol has the sharpened ninth. Other more uncommon notations and names include major/minor or 7 (add min 3). Kenn Stephenson says that in rock music the sharp ninth spelling is much more prevalent than the split third version.

The 7♯9 is an altered chord, and it is one option when seeing the chord symbol 7alt. It is functionally a dominant chord and thus "wants" to resolve to the tonic in diatonic harmony. Stuart Isacoff has called the chord "funky" or "bluesy" because of the tension "generated between the major third and the augmented ninth"; while Doug Munro deems it "jazzy" rather than bluesy. Eric Starr says, "the sharp nine tends to be edgier, bluesier, and meaner sounding [than the flat nine]." In jazz, 7♯9 chords, along with 7♭9 chords, are often employed as the dominant chord in a minor ii–V–I turnaround. For example, a ii–V–I in C minor could be played as: Dm^7♭5 – G^7♯9 – Cm⁷.

The 7♯9 represents a major divergence from the world of tertian chord theory, where chords are stacks of major and minor thirds. The 7♯9 does not satisfy that definition, as the interval between the minor seventh and augmented ninth is an augmented third. The same also pertains to the rarer M7♭9, where the interval between the major seventh and minor ninth is a diminished third. Rather than fully allow the inclusion of diminished and augmented thirds into the theory, a typical solution in jazz is to define chords as stacks of chordal degrees, where each degree has some range of selection from which to take its note or notes. Thus, for example, the ninth is available in flat, natural, sharp, and flat-and-sharp "alt" styles.

The English cadence is a distinctive contrapuntal pattern particular to the authentic or perfect cadence described as archaic or old-fashioned sounding. This pattern was primarily used by English composers of the High Renaissance and Restoration periods. The hallmark of this device is the dissonant augmented octave (compound augmented unison) produced by a false relation between the split seventh scale degree.

The English cadence is a type of full close featuring the blue seventh against the dominant chord which in C would be B♭ and G-B♮-D.