  Methyl mercaptan (also known as methanethiol), CH3SH, has been found in the
warm and dense parts of high -- as well as low -- mass star-forming regions.
The aim of the present study is to obtain accurate spectroscopic parameters of
the S-deuterated methyl mercaptan CH$_3$SD to facilitate astronomical
observations by radio telescope arrays at (sub)millimeter wavelengths. We have
measured the rotational spectrum associated with the large-amplitude internal
rotation of the methyl group of methyl mercaptan using an isotopically enriched
sample in the 150-510 GHz frequency range using the K\"oln millimeter wave
spectrometer. The analysis of the spectra has been performed up to the second
excited torsional state. We present modeling results of these data with the
RAM36 program. CH$_3$SD was searched for, but not detected, in data from the
Atacama Large Millimeter/submillimeter Array (ALMA) Protostellar
Interferometric Line Survey (PILS) of the deeply embedded protostar IRAS
16293-2422. The derived upper limit corresponds to a degree of deuteration of
at most ~18%.
