Three dual Lewis base functionalized ionic liquids (DLB-ILs) tethered with both carboxyl and tertiary amine groups on their anions were designed for highly efficient and selective absorption of H2S. It was found that the DLB-ILs could reversibly absorb 0.39–0.84 mol of H2S per mole of IL at 1 bar and 60 °C, which is significantly higher than that of other ILs. On the contrary, the CO2 absorption in this class of DLB-ILs was shown to be quite limited relative to H2S owing to the coupling effect of the two Lewis base groups. It is the dual Lewis base functionalization that enables the significantly high values calculated for the ideal absorption selectivity for H2S/CO2, that is, 13–26 at 1 bar and 29–70 at 0.1 bar (60 °C). The selectivity even goes up to >100 at low pressures and high temperatures. It was further illustrated from DFT calculations and spectroscopy studies that the cooperation interaction of carboxyl⋅⋅⋅H2S⋅⋅⋅amine and the reduced binding with CO2 were the major contributions to the high H2S absorption capacity and H2S/CO2 selectivity.