Hitherto unknown dithioformic acid, HCSSH, has been produced by pyrolysis (300°C) of methanetrithiol, HC(SH)₃, and identified by the microwave spectrum (18-40 GHz) of its most abundant species H¹²C³²S³²SH. Rotational transitions of this species in its ground state have been assigned to a trans and a cis rotamer. Both rotamers are planar, the trans conformer being the more stable by ca. 350 cm^-1 mole^-1. For each rotamer spectra of molecules in two vibrationally excited states have been assigned. Frequencies of transitions of importance for search for HCSSH in interstellar space are reported.