Thiosulfuric acid is the inorganic compound with the formula H2S2O3.

It has attracted academic interest as a simple, easily accessed compound that is labile.

[6] Anhydrous methods of producing the acid were developed by Max Schmidt:[6][7] The anhydrous acid also decomposes above −5 °C:[6] The isomer (O=)2S(−OH)(−SH) is more stable than the isomer (O=)(S=)S(−OH)2 as established by Hartree–Fock/ab initio calculations with a 6-311 G** basis set and MP2 to MP4 refinements.

[8][clarification needed] The theoretically predicted structure conforms with the double bond rule.

An isomer of thiosulfuric acid is the adduct of hydrogen sulfide and sulfur trioxide, H2S·SO3, which can also be prepared at low temperature.