Catgut is the name applied to cord of great toughness and tenacity prepared from the intestines of sheep/goat, or occasionally from those of the hog, horse, mule, and donkey. Those of the cat are not employed, and therefore it is supposed that the word is properly kitgut ("violin string"), kit meaning "fiddle," and that the present form has arisen through confusion with kit = cat.

The substance is used for the strings of harps, violins, and viols, as well as other stringed musical instruments, for hanging the weights of clocks, for bow-strings, and for suturing wounds in surgery. Catgut formerly was also used for stringing racquets.

To prepare it, the intestines are cleaned, freed from fat, and steeped for some time in water, after which their external membrane is scraped off with a blunt knife. They are then steeped for some time in an alkaline lye, smoothed and equalized by drawing out, subjected to the antiseptic action of the fumes of burning sulphur, if necessary dyed, sorted into sizes, and twisted together into cords of various numbers of strands according to their uses. The best strings for musical instruments are reputedly from Italy ("Roman strings"); and it is found that lean and ill-fed animals yield the toughest gut.

Though catgut was in use for producing strings for many centuries, its use in the medical field became popular only in the 19th century. It came as a replacement for the silk and hemp sutures which caused inflammation and severe hemorrhage because the body could not absorb it. Sutures made from catgut are readily absorbed by the human body and are thus extensively used for internal stiches. Though synthetic alternatives have been on the market, catgut sutures are still widely used in hospitals throughout the world.