  We report the detection in TMC-1 of the protonated form of C3S. The discovery
of the cation HC3S+ was carried through the observation of four harmonically
related lines in the Q band using the Yebes 40m radiotelescope, and is
supported by accurate ab initio calculations and laboratory measurements of its
rotational spectrum. We derive a column density N(HC3S+) = (2.0 +/- 0.5)e11
cm-2, which translates to an abundance ratio C3S/HC3S+ of 65 +/- 20. This ratio
is comparable to the CS/HCS+ ratio (35 +/- 8) and is a factor of about ten
larger than the C3O/HC3O+ ratio previously found in the same source. However,
the abundance ratio HC3O+/HC3S+ is 1.0 +/- 0.5, while C3O/C3S is just 0.11. We
also searched for protonated C2S in TMC-1, based on ab initio calculations of
its spectroscopic parameters, and derive a 3sigma upper limit of N(HC2S+) <
9e11 cm-2 and a C2S/HC2S+ > 60. The observational results are compared with a
state-of-the-art gas-phase chemical model and conclude that HC3S+ is mostly
formed through several pathways: proton transfer to C3S, reaction of S+ with
c-C3H2, and reaction between neutral atomic sulfur and the ion C3H3+.
