In music theory, an augmented sixth chord contains the interval of an augmented sixth, usually above its bass tone. This chord has its origins in the Renaissance, was further developed in the Baroque, and became a distinctive part of the musical style of the Classical and Romantic periods.

Conventionally used with a predominant function (resolving to the dominant), the three most common types of augmented sixth chords are usually called the Italian sixth, the French sixth, and the German sixth.

The augmented sixth interval is typically between the sixth degree of the minor scale, ♭, and the raised fourth degree, ♯. With standard voice leading, the chord is followed directly or indirectly by some form of the dominant chord, in which both ♭ and ♯ have resolved to the fifth scale degree, . This tendency to resolve outwards to is why the interval is spelled as an augmented sixth, rather than enharmonically as a minor seventh (♭ and ♭).

Although augmented sixth chords are more common in the minor mode, they are also used in the major mode by borrowing ♭ of the parallel minor scale.

There are three main types of augmented sixth chords, commonly known as the Italian sixth, the French sixth, and the German sixth.

Though each is named after a European nationality, theorists disagree on their precise origins and have struggled for centuries to define their roots, and fit them into conventional harmonic theory. According to Kostka and Payne, the other two terms are similar to the Italian sixth, which, "has no historical authenticity-[being] simply a convenient and traditional label."

The Italian sixth (It⁺⁶ or It⁶ or ♯iv⁶) is derived from iv⁶ with an altered fourth scale degree, ♯. This is the only augmented sixth chord comprising just three distinct notes; in four-part writing, the tonic pitch is doubled.

The Italian sixth is enharmonically equivalent to an incomplete dominant seventh. ♭VI7=♯V7: A♭, C, (E♭,) G♭.