Isocyanic acid is a chemical compound with the structural formula HNCO, which is often written as H−N=C=O. It is a colourless, volatile and poisonous substance, with a boiling point of 23.5 °C. It is the predominant tautomer and an isomer of cyanic acid (aka. cyanol) (H−O−C≡N).

The derived anion of isocyanic acid is the same as the derived anion of cyanic acid, and that anion is [N=C=O]⁻, which is called cyanate. The related functional group −N=C=O is isocyanate; it is distinct from cyanate (−O−C≡N), fulminate (−O−N⁺≡C⁻), and nitrile oxide (−C≡N⁺−O⁻).

Isocyanic acid was discovered in 1830 by Justus von Liebig and Friedrich Wöhler.

Isocyanic acid is the simplest stable chemical compound that contains carbon, hydrogen, nitrogen, and oxygen, the four most commonly found elements in organic chemistry and biology. It is the only fairly stable one of the four linear isomers with molecular formula HOCN that have been synthesized, the others being cyanic acid (cyanol, H−O−C≡N) and the elusive fulminic acid (H−C≡N⁺−O⁻) and isofulminic acid H−O−N⁺≡C⁻.