  In the $\mu\nu$SSM, the presence of $R$-parity violating couplings involving
right-handed (RH) neutrinos solves simultaneously the $\mu$- and
$\nu$-problems. We explore extensions of the $\mu \nu$SSM adding a $U(1)'$
gauge group, which provides the RH neutrinos with a non-vanishing charge. In
these models, dubbed U$\mu\nu$SSM, the anomaly cancellation conditions impose
the presence of exotic quarks in the spectrum that are vector-like under the
standard model (SM) gauge group: either three pairs $SU(2)$ quark singlets, or
a pair of quark singlets together with a pair of quark doublets. Several extra
singlets under the SM group can also be present, with the $U(1)'$ charges
making distinctions among them, and therefore allowing different types of
couplings. Some of these singlets dynamically generate Majorana masses for the
RH neutrinos, and others can be candidates for dark matter. The useful
characteristics of models with $U(1)'$s are also present in U$\mu\nu$SSM
models: baryon-number-violating operators as well as explicit Majorana masses
and $\mu$ terms are forbidden, and the domain wall problem is avoided. The
phenomenology of U$\mu\nu$SSM models is very rich. We analyze the experimental
constraints on their parameter space, specially on the mass and mixing of the
new $Z'$ boson. In addition to the exotic quarks, which can hadronize inside
the detector or decay producing SM particles, the U$\mu\nu$SSM models can also
have new signals such as decays of the $Z'$ to sparticle pairs like right
sneutrinos, charginos or neutralinos. Besides, $Z'$ and Higgs mediated
annihilations and interactions with the visible sector of WIMP dark matter
particles, can also be present.
