The multiple anion superlattice Bi4O4SeCl2has been reported to exhibit extremely low thermal conductivity along the stackingc-axis, making it a promising material for thermoelectric applications. In this study, we investigate the thermoelectric properties of Bi4O4SeX2(X = Cl, Br) polycrystalline ceramics with different electron concentrations by adjusting the stoichiometry. Despite optimizing the electric transport, the thermal conductivity remained ultra-low and approached the Ioffe–Regel limit at high temperatures. Notably, our findings demonstrate that non-stoichiometric tuning is a promising approach for enhancing the thermoelectric performance of Bi4O4SeX2by refining its electric transport, resulting in a figure of merit of up to 0.16 at 770 K.