Naphthalene-1-sulfonic acid is an organic compound with the formula C10H7SO3H.

A colorless, water-soluble solid, it is often available as the dihydrate C10H7SO3H.2H2O.

[1] Naphthalene-1-sulfonic acid undergoes many reactions, some of which are or were of commercial interest.

Upon heating with dilute aqueous acid, it reverts to naphthalene.

Fusion with sodium hydroxide followed by acidification gives 1-naphthol.