Johnny St. Cyr () (April 17, 1890 – June 17, 1966) was an American jazz banjoist and guitarist. He was one of the original pioneers of jazz music, playing banjo and guitar in the bands of Louis Armstrong, King Oliver, Jelly Roll Morton, Johnny Dodds and Kid Ory, among others. He started the idea of banjo with jazz, a combination whose impact changed the banjo world during the Jazz years, and continues to have a marked affect. He is best known for writing the songs "Messin' Around", "Buddy's Habit", "High Fever" and "Oriental Strut", and for playing the banjo and/or guitar on the Louis Armstrong Hot Five and Hot Seven Sessions.

St. Cyr played a Martin guitar, a four-string tenor banjo, or a six-string banjo guitar, making his own instrument by attaching the neck of a guitar to a banjo. The result was a louder banjo with a jazz tone; his sound is notably different from other banjo music recorded in that era. He used a hard, thick pick to produce the smooth rhythm guitar sound and hard, clean single string lines that typified his style–he made his own picks from toothbrush handles.

A hard-working, selfless bandsman, St. Cyr had a professional, simple outlook on musicianship: “A jazz musician has to be a working class of a man, out in the open all the time, healthy and strong," he commented. "Playing music for him is just relaxing. He gets as much kick out of playing as other folks get out of dancing. The more enthusiastic his audience is, the more spirit the working man’s got to play.”