  The U$\mu\nu$SSM is a $U(1)'$ extension of the $\mu\nu$SSM supersymmetric
model, where baryon-number-violating operators as well as explicit mass terms
are forbidden, and the potential domain wall problem is avoided. The gauge
anomaly-cancellation conditions impose the presence of exotic quark superfields
in the spectrum of U$\mu\nu$SSM models, and allow the presence of several
singlet superfields under the standard model gauge group, in addition to the
right-handed neutrino superfields. The gauge structure implies an additional
discrete $Z_2$ symmetry in the superpotential, ensuring the stability of a
singlet which behaves as WIMP dark matter without invoking $R$-parity. We
analyze this novel possibility in detail, using the fermionic component of the
singlet as the dark matter candidate. In particular, we compute its amount of
relic density via $Z'$, Higgs-right sneutrino and dark matter mediated
annihilations, and its potential signals in dark matter direct detection
experiments. The constraints on the parameter space due to $Z'$ direct searches
at the LHC are imposed in the analysis, as well as those from the hadronization
inside the detector of the exotic quarks. Large regions of the parameter space
turn out to be in the reach of the upcoming Darwin experiment.
