Reactive dye is a class of highly coloured organic substances, primarily utilised for tinting textiles, that attach themselves to their substrates by a chemical reaction that forms a covalent bond between the molecule of dye and that of the fibre. The dyestuff thus becomes a part of the fibre and is much less likely to be removed by washing than are dyestuffs that adhere by adsorption.

The most important characteristic of reactive dyes is the formation of covalent bonds with the substrate to be colored, i.e. the dye forms a chemical bond with cellulose, which is the main component of cotton fibers.