In a pipe organ, a string stop is an organ stop that uses a single rank of narrow, conical or tapered flue pipes to produce a bright sound, rich in harmonics. The pipes may be either open or stopped, and of either metal or wood, see organ pipe.

String tone is one of the four basic foundation sound families of a pipe organ, the others being diapason, flute , and reeds . It is available on almost all pipe organs. If one was to have only two ranks, the first would be a diapason, and the second most probably a string. Even if a stopped diapason or flute was the second choice, for choir work perhaps, the third would then be a string.

Although it takes some imagination to liken the sounds these pipes make to those of bowed string instruments, these stops are generally named after such instruments.

Examples:

• Bass viol.
• Cello.
• Gamba (presumably short for viol de gamba).
• Geigen, from the German geige for violin.
• Viol.