Dithionous acid is a sulfur oxoacid with the chemical formula H<sub>2</sub>S<sub>2</sub>O<sub>4</sub>. It has not been observed experimentally. It is the conjugate acid of the dithionite dianion, S<sub>2</sub>O<sub>4</sub><sup>2-</sup>. Dithionite is a well-known reducing agent.