Thiolactic acid is the organosulfur compound with the formula CH3CH(SH)CO2H.

The molecule contains both carboxyl and thiol functional groups, −C(=O)−OH and −SH respectively.

It is structurally related to lactic acid by the interchange of −SH for −OH.

Thiolactic acid was once widely used in hair permanent waving formulations, but has been displaced by formulations based on thioglycolic acid.

Instead of using the acid itself, its salts (2-sulfanylpropanoates) are used.