口琴（英语：Harmonica），用嘴吹气或吸气，使金属簧片振动发声的多簧片乐器；在乐器分类上属于自由簧的吹奏乐器。由于发声源是长度介于1.5～3.5公分的簧片而非空气柱（如长笛），独奏用口琴体积多在20×6×4厘米（长；宽；高）内，其音域依种类、调性不同而略有差异，大致可归纳成由中央C下1个八度至中央C上3个八度，涵盖约3至4个八度。

多簧片的结构，使口琴具有演奏和声的基础。对一般成年人而言，只要排列方向相同（同为吹气发声或吸气发声）且琴格间距在6公分以内的簧片均能同时发声；由于演奏中途无法调整同一把口琴上簧片的相对位置，单一把口琴演奏合声的能力是受限制的。一般而言，由乐器公司量产的口琴仅在八度和声方面较不受限制。

The harmonica, also French harp, and mouth organ, is a free reed wind instrument used worldwide in many musical genres, notably in blues, American folk music, classical music, jazz, country, and rock and roll. There are many types of harmonica, including diatonic, chromatic, tremolo, octave, orchestral, and bass versions. A harmonica is played by using the mouth (lips and tongue) to direct air into or out of one or more holes along a mouthpiece. Behind each hole is a chamber containing at least one reed. A harmonica reed is a flat elongated spring typically made of brass, stainless steel, or bronze, which is secured at one end over a slot that serves as an airway. When the free end is made to vibrate by the player's air, it alternately blocks and unblocks the airway to produce sound.