Dithionous acid is a sulfur oxoacid with the chemical formula H₂S₂O₄. It is unstable in pure form, but its salts, known as dithionites, are stable.

It was initially assumed that the C₂ symmetric structure HOS(=O)-S(=O)OH is the most stable among molecules with the formula H
2S
2O
4 using ab initio calculations. The reason for this is the existence of intermolecular hydrogen bonds. It is now known that dithionous acid spontaneously decomposes to SO₂ and S(OH)₂.