The marimba is a musical instrument in the percussion family. Keys or bars (usually made of wood) are struck with mallets to produce musical tones. The keys are arranged as a pianos are, with accidentals raised vertically and overlapping the 'regular' keys to aid the performer both visually and physically.

The marimba is pitched an octave lower than its cousin, the xylophone. Both xylophone and marimba bars are usually made of rosewood, but the bars of the marimba are wider and thicker than those of the xylophone, which gives the instrument a richer tone. Whereas the xylophone's key widths are constant along its entire length, modern marimba keys are usually short (both lengthwise and widthwise) at the higher pitched end and gradually expand into the bottom octaves. This ensures that larger marimbas, such as 5 octaves, have enough material to generate low notes and overtones.

Most marimba mallets are actually xylophone mallets with several layers of yarn wrapped around them. Since xylophone keys are smaller and thus harder, keylon, plastic, acrylic, wood, and other materials serve as ends of xylophone mallets, each having a characteristic attack and sound, attached to a long, thin piece of wood. Each of these mallets are then covered with different types of yarn and thicknesses to generate various timbres from the marimba. Typically, softer and thicker mallets are used on bottom octaves and harder, thinner mallets used on the top octaves. Mallet choice is typically left up to the performer, though some compositions specify if a certain sound is desired by the composer.

Modern marimba music calls for a range between two and six mallets to be used simultaneously. This allows for much wider range of musical styles, especially for solo performances. When more than two mallets are needed at once, two mallets can be held in the same hand using several methods, most notably the Steven's grip (invented by Leigh Stevens). With the wrist turned so that the top of the thumb is parallel with the ceiling, one mallet is placed on top of the index finger with the end inserted into the pocket generated by the thumb, and the second is placed on the top of the ring finger between the joints closest to the palm. Rotation of the hand either way allows the respective mallet to strike a certain key, where the interval of the mallets can be controlled primarily by curling of the index finger or extension of the thumb. To strike both mallets at the same time, flex the wrist up and then down in a concerted motion. Beginners typically complain of pain in the wrist for the first month or so because of the unusual grip and striking strategy. However, the Steven's grip has revolutioned marimba compositions since it allows a much greater degree of interval freedom and mallet control than other methods.

Key to the marimba's rich sound are its resonators. These are metal tubes below each bar, the length varying according to the pitch of the note. The bottom end of each resonator is closed, and the column of air inside resonates and amplifies the sound of the vibrating bar above. In large instruments (typically above 4 1/2 octaves) the length of tubing required for the bass notes exceeds the height of the instrument and the resonators are bent at the bottom.

Modern marimba uses include solo performances, percussion ensembles, marimba concertos, and, least often, wind ensemble or orchestra compositions.

The term marimba is also used to refer to various traditional folk instruments, the precursors of which may have developed independently in West Africa and in Pre-Columbian MesoAmerica. In the most traditional versions, various sizes of natural gourds are attached below the keys to act as resonators; in more sophisticated versions carved wooden resonators are substituted, allowing for more precise tuning of pitch. Traditional marimba bands are popular in Guatemala and parts of the highlands of southern Mexico; gyil duets are the traditional music of Dagara funerals in Ghana.