A multi-instrumentalist is a musician who plays two or more musical instruments at a professional level of proficiency.

Also known as doubling, the practice allows greater ensemble flexibility and more efficient employment of musicians, where a particular instrument may be employed only briefly or sporadically during a performance. Doubling is not uncommon in orchestra (e.g., flutists who double on piccolo) and jazz (saxophone/flute players); double bass players might also perform on electric bass. In music theatre, a pit orchestra's reed players might be required to perform on multiple instruments. Church piano players are often expected to play the church's pipe organ or Hammond organ as well.

In popular music it is more common than in classical or jazz for performers to be proficient on instruments not from the same family, for instance to play both guitar and keyboards. Many bluegrass musicians are multi-instrumentalists. Some musicians' unions or associations specify a higher rate of pay for musicians who double on two or more instruments for a performance or recording.