Nitrogen and sulfur codoped and completely renewable carbons were synthesized from two types of algae, Spirulina Platensis and Chlorella Vulgaris, without any additional nitrogen fixation reaction. The type of activation agents, char-forming temperature, activation agent-to-char ratio, and activation temperature were all varied to optimize the reaction conditions for this synthesis. The maximum Brunauer–Emmett–Teller surface area and total pore volumes of the carbons were 2685 m2/g and 1.4 cm3/g, respectively. The nitrogen and sulfur contents of the carbons were in the range of 0.9–5.69 at. % and 0.05–0.2 at. %, respectively. The key nitrogen functionalities were pyridinic, amino, and pyridonic/pyrrolic groups, whereas the key sulfur functionalities were S–C, O–S–C, and SOx groups. CO2 adsorption isotherms were measured at 273, 298, and 313 K, and the ideal adsorbed solution theory was employed to calculate the selectivity of adsorption of CO2 over N2 and simulate binary adsorption isotherms. The adsorption results demonstrated that the CO2 adsorption amount and the heat of CO2 adsorption were higher for carbons with higher nitrogen content, confirming the influence of nitrogen functionality in CO2 adsorption. The overall results suggested that these algae-derived renewable carbons can serve as potential adsorbents for CO2 separation from N2.