The lap steel guitar, also known as a Hawaiian guitar or lap slide guitar, is a type of steel guitar without pedals that is typically played with the instrument in a horizontal position across the performer's lap. Unlike the usual manner of playing a traditional acoustic guitar, in which the performer's fingertips press the strings against frets, the pitch of a steel guitar is changed by pressing a polished steel bar against strings while plucking them with the opposite hand. The steel guitar's name is derived from this steel bar. Though the instrument does not have frets, it displays markers that resemble them. Lap steels may differ markedly from one another in external appearance, depending on whether they are acoustic or electric, but in either case, they do not have pedals, distinguishing them from pedal steel guitars.

The steel guitar came from Hawaiʻi and gained a foothold in mainland American popular music. It originated in the Hawaiian Kingdom about 1885, popularized by an Oahu youth named Joseph Kekuku, who became known for playing a traditional guitar by laying it across his lap and sliding a piece of metal against the strings to change the pitch. The instrument's distinctive portamento sound, characterized by a smooth gliding between notes, became popular throughout the islands. American popular culture became fascinated with Hawaiian music during the first half of the twentieth century – to the degree of becoming a musical fad. Americans were curious about the lap steel instrument featured in its performance, and came to refer to it as a "Hawaiian guitar", and the horizontal playing position as "Hawaiian style". Hawaiian music began its assimilation into American popular music in the 1910s, but with English lyrics, a combination Hawaiians called hapa haole (half-white). In the 1930s, the invention of electric amplification for the lap steel was a milestone in its evolution. It meant that the instrument could be heard equally with other instruments, that it no longer needed a resonance chamber to produce its sound, and that electrified lap steels could be manufactured in any shape (even a rectangular block), with little or no resemblance to a traditional guitar.

In the early twentieth century Hawaiian music and the steel guitar began to meld into other musical styles, including blues, jazz, gospel, country music and, particularly, the country music sub-genres Western swing, honky-tonk, and bluegrass. Lap steel pioneers include Sol Hoopii, Bob Dunn, Jerry Byrd, Don Helms, Bud Isaacs, Leon McAuliffe, Josh Graves, Pete Kirby, and Darick Campbell.

Conceptually, a lap steel guitar may be likened to playing a guitar with one finger (the bar). This abstraction illustrates one of the instrument's major limitations: its constraint to a single chord that is not changeable during a performance without re-tuning the instrument. An early solution was to build lap steel guitars with two or more necks, each providing a separate set of differently-tuned strings on a single instrument. The performer's hands could move to a different neck at will. Although in the early 1940s, elite players recorded and performed with these multi-neck guitars, most musicians could not afford them. The problem was addressed in 1940 by adding pedals to the lap steel to change the pitch of certain strings easily, making more complex chords available on the same neck. By 1952, this invention revolutionized how the instrument was played, in many ways making it virtually a new instrument, known as a "pedal steel". An overwhelming majority of lap steel players adopted the pedal design, and, as a result, the lap steel became largely obsolete by the late 1950s, with only pockets of devotees in country and Hawaiian music remaining.

Spanish guitars were introduced into the Hawaiian Islands as early as the 1830s. The Hawaiians did not embrace the standard guitar tuning that had been in use for centuries. They re-tuned the guitars to make a chord when all the strings were sounded together, known as an "open tuning". This was called "slack-key", known in Hawaiian as "kī hōʻalu", because certain strings were "slackened" to achieve it. Hawaiians learned to play fingerstyle this way, creating melodies over the full resonant tones of the open strings, and the genre became known as slack-key guitar. About 1885, after guitar strings made of steel became available, Joseph Kekuku, on the island of Oahu, developed and popularized playing an open tuning while seated with the guitar across his knees while pressing a steel bar against the strings. Following Kekuku's lead, other Hawaiians began playing in this new manner, with the guitar laid across the lap, instead of in the traditional way of holding the instrument against the body. Once the horizontal style became popular throughout the islands, the technique spread internationally, and was referred to (typically outside of Hawaii) as "Hawaiian style".

Hawaiian music, with the sound of the steel guitar as a marked feature, became a popular musical preoccupation or fad in the United States in the first half of the twentieth century. In 1916, recordings of indigenous Hawaiian music outsold all other U.S. musical genres. This popularity initiated the manufacture of guitars designed specifically to be played horizontally. The archetypal lap steel guitar is the acoustic Hawaiian guitar. Despite incorporating a resonant chamber in their body, these early acoustic versions of the instrument were not loud enough relative to other instruments. However, in the early 1930s a steel guitarist named George Beauchamp invented the electric guitar pickup. Electrification not only allowed the lap steel guitar to be heard better, but it also meant that their resonance chambers were no longer essential. The result was that steel guitars could be manufactured in any shape – even in the form of 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".

There are three categories of lap steel guitars:

Over centuries in Western countries, the traditional Spanish guitar developed a near-universal tuning of ascending fourths (and one major third) consisting of E–A–D–G–B–E; however, no such standard existed for the Hawaiian "open tunings" (guitar tuned in a chord). The Hawaiians simply tuned to a chord that suited the singer's voice. Beginning in the days of slack-key guitar in the 1850s, Hawaiian tunings came to be as closely guarded as any trade secret, handed down in families. Many players de-tuned their instruments when they were not playing them to keep others from discovering their tuning.