The title acid, C4H5NO2S2, crystallized in the non-centrosymmetric space group P212121 with one molecule as the asymmetric unit. Two hydrogen bonds occur, namely, N-HO, with a donor-acceptor distance of 2.850 (2) Å, and O-HS (resonance induced), with a donor-acceptor distance of 3.318 (2) Å; both H atoms involved are ordered. These two types of bonds link a given molecule to four neighbors, the molecules linked being of space-group symmetry types 1 and 3, or 2 and 4. These two subsets of molecules alternate in layers along c. Three significant C-HO interactions occur, one of which crosslinks the subsets just described to produce a three-dimensional network of linked molecules. The absolute structure is determined.