A free reed wind instrument with multiple, variably-tuned brass or bronze reeds which are secured at one end over an airway slot in which they can freely vibrate. The vibrating reeds repeatedly interrupt the airstream to produce sound.

Unlike most free reed instruments (such as reed organs, accordions and melodicas), the harmonica lacks a keyboard. Instead, the player selects the notes by placing the mouth over the proper airways.

These holes are usually discrete holes in the front of the instrument. Each hole communicates with one or more reeds, depending on the type of harmonica. Because a reed mounted above a slot is made to vibrate more easily by air from above, reeds accessed by a mouthpiece hole often may be selected further by choice of breath direction (blowing, drawing).

Some harmonicas, primarily the chromatic harmonica, also include a spring-loaded button-actuated slide that, when depressed, redirects the airflow.

The harmonica was invented in 1821 and is used mostly in folk and blues music.