String instruments, stringed instruments, or chordophones are musical instruments that produce sound from vibrating strings when a performer plays or sounds the strings in some manner.

Musicians play some string instruments by plucking the strings with their fingers or a plectrum—and others by hitting the strings with a light wooden hammer or by rubbing the strings with a bow. In some keyboard instruments, such as the harpsichord, the musician presses a key that plucks the string. Other musical instruments generate sound by striking the string.

With bowed instruments, the player pulls a rosined horsehair bow across the strings, causing them to vibrate. With a hurdy-gurdy, the musician cranks a wheel whose rosined edge touches the strings.

Bowed instruments include the string section instruments of the orchestra in Western classical music (violin, viola, cello and double bass) and a number of other instruments (e.g., viols and gambas used in early music from the Baroque music era and fiddles used in many types of folk music). All of the bowed string instruments can also be plucked with the fingers, a technique called "pizzicato". A wide variety of techniques are used to sound notes on the electric guitar, including plucking with the fingernails or a plectrum, strumming and even "tapping" on the fingerboard and using feedback from a loud, distorted guitar amplifier to produce a sustained sound. Some string instruments are mainly plucked, such as the harp and the electric bass. In the Hornbostel-Sachs scheme of musical instrument classification, used in organology, string instruments are called chordophones. Other examples include the sitar, rebab, banjo, mandolin, ukulele, and bouzouki.

According to Sachs,

Chordophones are instruments with strings. The strings may be struck with sticks, plucked with the bare fingers or a plectrum, bowed or (in the Aeolian harp, for instance} sounded by wind. The confusing plenitude of stringed instruments can be reduced to four fundamental type: zithers, lutes, lyres, and harps.

In most string instruments, the vibrations are transmitted to the body of the instrument, which often incorporates some sort of hollow or enclosed area. The body of the instrument also vibrates, along with the air inside it. The vibration of the body of the instrument and the enclosed hollow or chamber make the vibration of the string more audible to the performer and audience. The body of most string instruments is hollow. Some, however—such as electric guitar and other instruments that rely on electronic amplification—may have a solid wood body.