D'Addario is a manufacturer of musical instrument strings, primarily for guitars, currently headquartered in Farmingdale, Long Island, New York. It is a family-owned and -operated business that is one of the largest string manufacturers in the world, not only producing several lines of strings under their own brand names, but also making OEM strings for other musical instrument companies.

D'Addario also produces and distributes other musical accessories under other brands. Some of the products offered are Cables, capos, ear plugs, electronic tuners, picks, straps, humidifiers, picks, slides and other accessories., drumheads, drum sticks and reeds for woodwind instruments.

The D'Addario family of string-makers originated in the small Italian town of Salle in the province of Pescara. A baptismal form from 1680 names a Donato D'Addario as a cordaro, where cordaro is a regional variant of Italian cordaio meaning "maker or seller of ropes and strings". From other historical records it appears that the town's primary occupations were farming and string-making. At the time strings were made of sheep or hog gut, and making them was a laborious process.

After an earthquake devastated the town in 1905, two brothers-in-law, Rocco and Carmine D'Addario emigrated to Astoria in Queens, New York in an attempt to expand their market, importing and selling the strings made by their family in Salle. By 1918 Rocco had returned to Salle, and Carmine, who later, known as Charles, began making his own strings in a small shop behind the family home. Still made from gut, the process of making strings involved all members of the family.

The guitar saw a major rise in popularity in the early part of the 20th century, because of new popular music, and sometime in the 1930s the family began making strings for this instrument, producing strings made to order for individual musicians or for guitar manufacturers. The development of nylon by DuPont during World War II produced a major change in the family business. Sent samples by Dupont in 1947, the D'Addarios immediately began experimenting with this new material, consulting with many of its regular customers in developing the strings.

During the late 1940s and early 1950s (especially after the birth of rock and roll) nylon-stringed "classical" guitars were being eclipsed in popularity by the steel-string guitar. Some of the younger members of the family wanted to expand into steel strings, but Charles was reluctant to risk the family business on what he considered an uncertain market. In 1956 a new company (the Archaic Musical String Mfg Co.) began to make steel strings, run by Charles' son, John D'Addario Sr. The company made strings for several of the major guitar makers of the time, including Gretsch, D'Angelico, Martin, and Guild. In 1962 the two companies were merged under the name Darco. The guitar had become the most popular instrument in the U.S., and the Darco company came up with many innovations in the manufacture of guitar strings, including the first automated equipment to wind strings and the first roundwound bass guitar strings. In the late 1960s, Darco was approached by Martin Guitars regarding a merger in order to pool resources and development efforts. While the partnership was beneficial for both companies, by 1974 the D'Addario family decided it was time to market strings under their own name, and the J. D'Addario & Company corporation was formed. Darco is still a brand name used by the Martin Guitar company. Originally located in Lynbrook, New York, the business continued to expand and in 1994 moved to its current facility in Farmingdale, New York. The company is still owned and operated by the D'Addario family, with 13 family members among the 1,000 employees of the company. Today D'Addario runing such brands as: D'Addario (Guitar and bass strings), Evans Drumheads (Drumheads), Planet Waves (Cables, capos, ear plugs, electronic tuners, picks, straps, humidifiers, picks, slides), ProMark (Drum sticks), Rico Reeds (Reeds).