Green S, Greens S, Food Green S, FD&C Green 4, Acid green 50, Lissamine Green B, Wool Green S, or C.I. 44090, is a green synthetic coal tar triarylmethane dye, C₂₇H₂₅N₂O₇S₂Na, usually a monosodium salt.

As a food dye, it has E number E142. It can be used in mint sauce, desserts, gravy granules, sweets, ice creams, and tinned peas.

Greens S is prohibited as a food additive in Canada, United States, Japan, and Norway. It may cause allergic reactions.

It is one of the colourants that the Hyperactive Children's Support Group recommends be eliminated from the diet of children.

It is a vital dye, meaning it can be used to stain living cells. It is used in ophthalmology, among fluorescein and rose bengal, to diagnose various disorders of the eye's surface.

Its CAS number is [3087-16-9] and its SMILES structure is [Na+].CN(C)c1ccc(cc1)C(=C2C=CC(C=C2)=[N+](C)C)c3c(O)c(OS([O-])=O)cc4cc(OS([O-])=O)ccc34.

Its risk and safety phrases are R22.