Engineered biochar derived from wood waste pyrolysis in molten salts were developed for effective CO2/N2 separation. The production conditions were customized to obtain the biochar with high CO2 capture capacity and CO2/N2 selectivity by tuning the type of molten salts (MgCl2-KCl, ZnCl2-KCl, ZnCl2-NaCl-KCl, and K2CO3-Na2CO3-Li2CO3), salt/feedstock ratios (1:1 and 3:1) and pyrolysis temperatures (600 and 800 °C). High temperature (800 °C) and moderate salt loading (salt/feedstock ratio of 1:1) benefited the CO2 adsorption by providing an increased surface area and highly dispersed metal species as adsorption sites. PSL-3-800 and PSL-3-600 (K2CO3-Na2CO3-Li2CO3 biochar) showed the highest CO2 capacity (4.5 mmol g−1, 0 °C, 100 kPa) and the highest CO2/N2 selectivity (28.5), respectively, among the engineered biochar developed in this study. In addition, ZP-3-600 showed the highest selection parameter (S) in both PSA and VSA processes, indicating the promising CO2 capture performance under PSA/VSA conditions. A high recovery rate (89%) of molten salts was achieved. These results suggest a new pathway for upcycling biowaste as eco-friendly and effective adsorbents for gas adsorption and separation.