Swing music, sometimes known as Swing jazz or just Swing, is a form of jazz music that developed in the early 1930s and had solidified as a distinctive style by 1935 in the United States and Europe. Swing uses a strong anchoring rhythm section which supports a lead section that can include brass instruments including saxophones, trumpets, and trombones or stringed instruments including violin and guitar; usually medium to fast tempos; and a distinctive swing rhythm. Swing bands usually featured soloists who would improvise variations on the melody over the arrangement. The danceable swing style of bandleaders such as Glenn Miller, Benny Goodman, Duke Ellington, and Count Basie was the dominant form of American popular music from 1935 to 1945.

The verb "to swing" is used as a term of praise for playing that expresses the distinctive rhythm characteristic of Swing.

Throughout the 1920s and early'30s, a dance-oriented form of jazz was popular. This danceable jazz used sweet and romantic melodies accompanied by lush orchestral arrangements. Orchestras tended to stick fairly close to the melody as written. Swing music did away with the large string sections and used simpler arrangements that usually emphasized horns and wind instruments (but sometimes violin or other strings) and improvised melodies.

Swing, like several other styles of 20th-century popular music, traces its origins to African rhythms. Traditional West African music brought to the US (and other places) by Africans (initially those captured and traded as slaves) eventually combined with western music to create a distinct style. Swing evolved out of the jazz experimentation that began in New Orleans and developed further in Kansas City, New York City, Paris, Buenos Aires, and a few other centres, and what is now called Swing diverged from other jazz music and established its own identity.

The popular jazz of the late teens through the late 1920s were most often played with two beat rhythms, and often attempted to reproduce the style of contrapuntal improvisation developed by the earliest generation of jazz musicians in New Orleans. In the late '20s, however, larger ensembles using written arrangements became more common, and a stylistic shift took place in the timing, which developed into a four beat rhythm with a syncopated way of playing the melody, while the rhythm section provided a steady four to the bar.

Those changes produced a more sophisticated sound than the styles of the '20s, and with a feel of its own that makes the listener want to dance. Most jazz bands had adopted this style by the early '30s, but older-style bands remained the more popular with white dancers until Benny Goodman appeared at the Palomar Ballroom in August of 1935. The audience of young white dancers could hardly get enough of Goodman's "hot" rhythms and daring swing arrangements. Swing (and Boogie Woogie) remained the dominant form(s) of American popular music for the succeeding ten years.

With the wider acceptance of Swing music from about 1935, larger mainstream bands began to embrace the style. Large orchestras reinvented themselves in order to produce the new sound. Many of these bands dropped their stringed instruments altogether, and nearly all eliminated their large string sections. Band leaders began putting more time and energy into developing arrangements, reducing the chaos that could result from as many as a dozen or more musicians improvising spontaneously.

In addition to the Swing bands mentioned above, notable exponents of Swing include the Hot Club of Paris (Stephane Grappelli, Django Rheinhardt, et al.), Josephine Baker's backup band of the same place and era, The duo (with other accompanying musicians) of Joe Venutti and Eddie Lang, Oscar Aleman's band in Buenos Aires, Louis Jordan's band, and others.

Since the late 1990s, Swing has enjoyed a resurgence of popularity worldwide, particularly in North America and Europe. Modern Swing groups leading the charge include the Squirrel Nut Zippers, Big Bad Voodoo Daddy, the Cherry Poppin' Daddies, Pendulum, the Royal Crown Revue, the Nairobi Trio, and the Budapest Swing Quartet.