In gas-phase adsorption processes, adsorbents are typically used as beads or pellets in a fixed bed. While these shapes facilitate their application in adsorption processes, they fall short of achieving optimal performance. This limitation can be greatly improved by employing structured materials, which offer reduced pressure drop and enhanced mass and energy transfer, thereby improving the overall process efficiency. Here, resol-based activated carbons (ACs) are structured using 3D-printed sacrificial templates to produce custom-designed monoliths for efficient carbon dioxide (CO2) capture. The influence of activation conditions on CO2 adsorption capacity and CO2/nitrogen (N2) selectivity are investigated. The surface area and micropore volume of the prepared 3D-structures, as well as their CO2 adsorption capacity, increase with activation time. However, the 3D structure with the shortest activation time has the highest selectivity for CO2 over N2 (considering a binary CO2/N2 mixture with 15 mol% of CO2), due to its lower N2 adsorption capacity. Overall, this work highlights the potential of modern 3D-printing techniques to produce custom-designed structured adsorbents with applications in gas separation processes, such as CO2 capture.