We report the gas-phase detection and spectroscopic characterization of ethynethiol ($mathrm{HCCSH}$), a metastable isomer of thioketene ($mathrm{H_2C_2S}$) using a combination of Fourier-transform microwave and submillimeter-wave spectroscopies. Several $a$-type transitions of the normal species were initially detected below 40 GHz using a supersonic expansion-electrical discharge source, and subsequent measurement of higher-frequency, $b$-type lines using double resonance provided accurate predictions in the submillimeter region. With these, searches using a millimeter-wave absorption spectrometer equipped with a radio frequency discharge source were conducted in the range 280 - 660 GHz, ultimately yielding nearly 100 transitions up to $^rR_0(36)$ and $^rQ_0(68)$. From the combined data set, all three rotational constants and centrifugal distortion terms up to the sextic order were determined to high accuracy, providing a reliable set of frequency predictions to the lower end of the THz band. Isotopic substitution has enabled both a determination of the molecular structure of $mathrm{HCCSH}$ and, by inference, its formation pathway in our nozzle discharge source via the bimolecular radical-radical recombination reaction $mathrm{SH + C_2H}$, which is calculated to be highly exothermic (-477 kJ/mol) using the HEAT345(Q) thermochemical scheme.
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