Abstract: A light vector boson, Z_d, associated with a "dark sector" U(1)_d gauge group, may help explain some astrophysical observations, as well as certain low energy laboratory anomalies. In such extensions of the Standard Model, the Higgs boson may have rare decays into X+Z_d, where X = Z, Z_d or gamma, mediated by mass-mixing or heavy new fermion loops. We examine the LHC phenomenology of these Higgs decays, focusing on Z_d masses of several GeV. Our estimates suggest that the rates are potentially measurable at the LHC, for interesting regions of parameter spaces, at a level complementary to low energy experimental searches for the Z_d. We also discuss how measurement of the Z_d polarization (longitudinal versus transverse) can be used to distinguish the physics underlying these rare decays.