Fiber reactive dyes are textile dyes which form a covalent bond with the fibers of the textile, resulting in a long lasting, bright dye. They can be challenging to work with and more expensive than conventional fabric dyes, but many crafters prefer them. Many craft stores sell fiber reactive dyes, as do some Internet retails, usually specifying which fibers the dye is designed to work with. In addition to the dye itself, salt and soda ash are necessary to set the dye.

The earliest fiber reactive dyes were designed to interact with cellulose fibers, such as cotton, linen, and hemp. When these materials are treated with soda ash to raise their pH, the fiber reactive dye interacts with the cellulose molecules, forming a permanent bond. Other fiber reactive dyes are designed for use with proteins like silk and wool, and some companies also offer variants for polymer based fabrics like polyester. After the clothing is rinsed and dried, the dye sets, and it will not come out.

Clothing colored with fiber reactive dyes will not bleed in washing or fade, unless exposed to bright light. This is why the dyes are popular for tie-dyes and batik, which often feature brightly colored patterns. Fiber reactive dyes can also be used for solid colors, or to paint on an assortment of textiles. The colorfastness and fade resistance of fiber reactive dyes makes them excellent choices for children's clothing and other textiles which need to be washed frequently.

Many consumers also appreciate the eco-friendliness of fiber reactive dyes. Some companies process the dyes with natural ingredients and materials, focusing on creating a product with a minimum of harmful waste. Since the dyes are colorfast, they will not bleed into wash water, leading to a reduction in dye-laden water runoff, which can be harmful for the environment. Fiber reactive dyes made with natural pigments tend to be less brilliant and distinctive, but they can still be quite beautiful.

To use a fiber reactive dye successfully, always pre-wash the textile being dyed, to remove any residual substances which may interact in a surprising way with the dye. Next, the dye should be mixed with non-iodized salt and stirred so that it is even before being strained to remove large particles. The fabric should be placed in a big tub with the dye mixture, and stirred regularly for approximately 20 minutes. Afterwards, the fabric is soaked in soda ash to set the dye, and then the mixture is rinsed before being hung to dry.