We report the gas-phase detection and spectroscopic characterization of ethynethiol(HCCSH), a metastable isomer of thioketene (H2C2S) using a combination of Fourier-transform microwave and submillimeter-wave spectroscopies. Severala-type transitionsof the normal species were initially detected below 40 GHz using a supersonic expansion-electrical discharge source, and subsequent measurement of higher-frequency,b-typelines using double resonance provided accurate predictions in the submillimeter region.With these, searches using a millimeter-wave absorption spectrometer equipped with aradio frequency discharge source were conducted in the range 280 – 660 GHz, ultimatelyyielding nearly 100 transitions up torR0(36) andrQ0(68). From the combined dataset, all three rotational constants and centrifugal distortion terms up to the sextic orderwere determined to high accuracy, providing a reliable set of frequency predictions tothe lower end of the THz band. Isotopic substitution has enabled both a determinationof the molecular structure of HCCSH and, by inference, its formation pathway in ournozzle discharge source via the bimolecular radical-radical recombination reaction SH+C2H, which is calculated to be highly exothermic (-477 kJ/mol) using the HEAT345(Q) thermochemical scheme