Thiophosphoric acid is an inorganic compound with the chemical formula H3PO3S.

Structurally, it is the acid derived from phosphoric acid with one oxygen atom replaced by sulfur atom, although it cannot be prepared from phosphoric acid.

It is a colorless compound that is rarely isolated in pure form, but rather as a solution.

The structure of the compound has not been reported, but two tautomers are reasonable: S=P(−OH)3 and O=P(−OH)2(−SH).

The compound has been prepared in a multistep process starting with the base hydrolysis of phosphorus pentasulfide to give dithiophosphate, which is isolated as its barium salt:[2] In a second stage, the barium salt is decomposed with sulfuric acid, precipitating barium sulfate and liberating free dithiophosphoric acid: Under controlled conditions, dithiophosphoric acid hydrolyses to give the monothioderivative: