We consider a concise dark matter scenario in the minimal gauged B − L extension of the standard model (SM), where the global B − L (baryon number minus lepton number) symmetry in the SM is gauged, and three generations of right-handed neutrinos and a B − L Higgs field are introduced. Associated with the B − L gauge symmetry breaking by a VEV of the B − L Higgs field, the seesaw mechanism for generating the neutrino mass is automatically implemented after the electroweak symmetry breaking in the SM. In this model context, we introduce a Z₂-parity and assign an odd parity for one right-handed neutrino while even parities for the other fields. Therefore, the dark matter candidate is identified as the right-handed Majorana neutrino with odd Z₂ parity, keeping the minimality of the particle content intact. When the dark matter particle communicates with the SM particles mainly through the B − L gauge boson (Z′⌄(BL) boson), its relic abundance is determined by only three free parameters, the B − L gauge coupling (α⌄(BL)), the Z′⌄(BL) boson mass (m⌄(Z')) and the dark matter mass (m⌄(DM)). With the cosmological upper bound on the dark matter relic abundance we find a lower bound on α⌄(BL) as a function of m⌄(Z′). On the other hand, we interpret the recent LHC Run-2 results on search for Z′ boson resonance to an upper bound on α⌄(BL) as a function of m⌄(Z′). Combining the two results we identify an allowed parameter region for this “Z′⌄(BL) portal” dark matter scenario, which turns out to be a narrow window with the lower mass bound of m⌄(Z′) > 2.5 TeV.