Yarn is a long continuous length of interlocked fibres, used in sewing, crocheting, knitting, weaving, embroidery, ropemaking, and the production of textiles. Thread is a type of yarn intended for sewing by hand or machine. Modern manufactured sewing threads may be finished with wax or other lubricants to withstand the stresses involved in sewing. Embroidery threads are yarns specifically designed for needlework. Yarn can be made of a number of natural or synthetic materials, and comes in a variety of colors and thicknesses (referred to as "weights"). Although yarn may be dyed different colours, most yarns are solid coloured with a uniform hue.

The word "yarn" comes from Middle English, from the Old English gearn, akin to Old High German garn, "yarn", Dutch garen, Ancient Greek χορδή (chordē, "string"), and Sanskrit hira, "band". It originally referred to entrails.

The human production of yarn is known to have existed since the Stone Age and earlier prehistory, with ancient fiber materials developing from animal hides, to reeds, to early fabrics. Cotton, wool, and silk were the first materials for yarn, and textile trade contributed immensely to the ancient global economy.

Yarn can be made from a number of natural or synthetic fibers, or a blend of natural and synthetic fibers.

The most common plant fiber is cotton, which is typically spun into fine yarn for mechanical weaving or knitting into cloth.

Silk is a natural protein fiber, some forms of which can be woven into textiles. The protein fiber of silk is composed mainly of fibroin and is produced by the larvae of the moth Bombyx mori. Silk production is thought to have begun in China and silk thread and cloth manufacture was well-established by the Shang dynasty (1600–1050 BCE).

Linen is another natural fiber with a long history of use for yarn and textiles. Linen fibers are derived from the flax plant.

Other plant fibers which can be spun include bamboo, hemp, maize, nettle, and soy fiber.