A steel guitar (Hawaiian: kīkākila) is any guitar played while moving a steel bar or similar hard object against plucked strings. The bar itself is called a "steel" and is the source of the name "steel guitar". The instrument differs from a conventional guitar in that it is played without using frets; conceptually, it is somewhat akin to playing a guitar with one finger (the bar). Known for its portamento capabilities, gliding smoothly over every pitch between notes, the instrument can produce a sinuous crying sound and deep vibrato emulating the human singing voice. Typically, the strings are plucked (not strummed) by the fingers of the dominant hand, while the steel tone bar is pressed lightly against the strings and moved by the opposite hand.

The idea of creating music with a slide of some type has been traced back to early African instruments, but the modern steel guitar was conceived and popularized in the Hawaiian Islands. The Hawaiians began playing a conventional guitar in a horizontal position across the knees instead of flat against the body, using the bar instead of fingers. Joseph Kekuku developed this manner of playing a guitar, known as "Hawaiian style", about 1890 and the technique spread internationally.

The sound of Hawaiian music featuring steel guitar became an enduring musical fad in the United States in the first half of the twentieth century and in 1916, recordings of indigenous Hawaiian music outsold all other U.S. musical genres. This popularity spawned the manufacture of guitars designed specifically to be played horizontally. The archetypal instrument is the Hawaiian guitar, also called a lap steel. These early acoustic instruments were not loud enough relative to other instruments, but that changed in 1934 when a steel guitarist named George Beauchamp invented the electric guitar pickup. Electrification allowed these instruments to be heard, and it also meant their resonant chambers were no longer essential. This meant steel guitars could be manufactured in any design, even a rectangular block bearing little or no resemblance to the traditional guitar shape. This led to table-like instruments in a metal frame on legs called "console steels", which were technologically improved about 1950 to become the more versatile pedal steel guitar.

In the United States, the steel guitar influenced popular music in the early twentieth century, combining with jazz, swing and country music to be prominently heard in Western swing, honky-tonk, gospel and bluegrass. The instrument influenced Blues artists in the Mississippi Delta who embraced the steel guitar sound but continued holding their guitar in the traditional way; they used a tubular object (the neck of a bottle) called a "slide" around a finger. This technique, historically called "bottleneck" guitar, is now known as "slide guitar" and is commonly associated with blues and rock music. Bluegrass artists adapted the Hawaiian style of playing in a resonator guitar known as a "Dobro", a type of steel guitar with either a round or square neck, sometimes played with the musician standing and the guitar facing upward held horizontally by a shoulder strap.