Naphthionic acid is an organic compound with the formula C₁₀H₆(NH₂)(SO₃H). It is one of several aminonaphthalenesulfonic acids, which are derivatives of naphthalene containing both amine and sulfonic acid functional groups. Naphthionic acid is a white solid, although commercial samples can appear gray. Deprotonation of naphthionic acid gives the sulfonate C₁₀H₆(NH₂)(SO₃)⁻, which is call naphthionate.

Naphthionic acid is prepared by treating 1-aminonaphthalene with sulfuric acid.

Naphthionic acid is used in the synthesis of azo dyes such as Rocceline (also known as Solid Red A). For this conversion sodium naphthionate is diazotated. The resulting diazonium derivative is coupled with β-naphthol.

Heating a solution of sodium naphtionate induces its conversion to 1-aminonaphthalene-2-sulfonate.