An acoustic guitar is a modern form of guitar descended from the Classical guitar, but generally strung with steel strings for a brighter, louder sound. Much heavier construction is required to withstand the added tension. Sometimes the term steel-stringed guitar or folk guitar is used to differentiate from the classical guitar. The term "acoustic guitar" is a retronym, since before the invention of the electric instrument, "guitar" meant only the non-amplified variety.

The acoustic guitar is often played with one or more plectra, or guitar picks, small plates of hard plastic or other material used to strike and strum the strings. Most players use a flat pick held between the thumb and index finger. A few use fingerpicks, picks worn on the thumb and three fingers. Fingerstyle acoustic guitar players use fingerpicks or use their bare fingers and fingernails to strum and strike individual notes.

There are many different variations on the construction of and materials used in acoustic guitars. More expensive guitars feature solid wood tops (usually Sitka spruce) and sides (often Indian rosewood, maple, or mahogany). Entry-level guitars are usually made of laminated wood.

There are several prominent American makers of acoustic guitars. Martin, Taylor, and Gibson are known for both the quality and price of their instruments.

The standard tuning for an acoustic guitar is EADGBE (low to high), although many players, particularly fingerpickers, use alternate tunings, such as "Open G" (DGDGBD), "Open D" (DADF#AD), or "Drop D" (DADGBE).

One variation on the standard acoustic guitar is the 12 string guitar, which sports an additional doubling string for each of the traditional six strings. This guitar was made famous by artists such as Huddie Leadbetter (a.k.a. Leadbelly), Pete Seeger, and Leo Kottke.