a bright greenish blue organic dye belonging to the phenothiazine family. It is mainly used on bast (soft vegetable fibres such as jute, flax, and hemp) and to a lesser extent on paper, leather, and mordanted cotton. It dyes silk and wool but has very poor lightfastness on these fibres. It is also employed as a biological stain, in testing milk for tubercular infection, and as a chemical oxidation–reduction indicator.

Methylene blue was discovered in 1876 and is manufactured by a process, introduced about 1886, using dimethylaniline as the principal starting material.