Sodium–bismuth–titanate (NBT) has recently been shown to contain high levels of oxide ion conductivity. Here we report the effect of A-site monovalent ions, M+ = K+ and Li+, on the electrical conductivity of NBT. The partial replacement of Bi3+ with monovalent ions improved the ionic conductivity by over one order of magnitude without an apparent change of the conduction mechanism, which is attributed to an increase in the oxygen vacancy concentration based on an acceptor-doping approach. The 18O tracer-diffusion coefficient (D*) determined by the isotope exchange depth profile method in combination with secondary ion mass spectrometry confirmed that oxygen ions are the main charge carriers in the system. Among these acceptor-doped samples, 4% Li doping provides the highest total conductivity, leading to a further discussion of doping strategies for NBT-based materials to enhance the electrical behavior, is discussed. Comparisons with other oxide-ion conductors and an oxygen-vacancy diffusivity limit model in perovskite lattice suggested that the doped NBT-based materials might already have achieved the optimization of the ionic conductivity.