  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.
