Spider silk is a protein fiber spun by spiders. Spiders use their silk to make webs or other structures, which function as nets to catch other animals, or as nests or cocoons for protection for their offspring. They can also suspend themselves using their silk.

Many small spiders use silk threads for ballooning, the popular, though technically inaccurate, scientific term for the dynamic kiting spiderlings (mostly) use for dispersal. They extrude several threads into the air and let themselves be carried away by upward winds. Although most rides will end a few yards later, it seems to be a common way for spiders to invade islands. Many sailors have reported that spiders have been caught in their ship's sails, even when far from land. The extremely fine silk used by spiders for ballooning is known as gossamer.

In some cases, spiders may even use silk as a source of food.

Methods have been developed to silk a spider forcibly.

All spiders produce silks, and a single spider can produce up to seven different types of silk for different uses. This is in contrast to insect silks, where most often only one type of silk is produced by an individual. Over the 400 million years of evolution, spider silks may be used for a number of different ecological uses, each with properties to match the function of the silk (see Properties section). The evolution of spiders has led to more complex and diverse uses of silk throughout its evolution, for example from primitive tube webs 300–400 mya to complex orb webs 110 mya.