As a notorious hazardous waste, biomass tar has for a long time been recognized as one of the key challenges throughout the progress in biomass gasification for renewable energy and chemical purposes. In this contribution, we report that biomass tar featured with considerably high carbon and nitrogen contents could serve as an ideal source for production of nitrogen-doped highly ultramicroporous carbon following the facile activation approach, which enabled a remarkably high ultramicroporosity based on a direct bottom-up strategy. Further, the disclosed evenly distributed active sites by nitrogen-doped ultramicropores displayed excellent CO2 absorption capacity as high as 6.02 and 4.11 mmol/g (1 bar) at 273 and 298 K, respectively, in addition to the corresponding ideal adsorption solvent theory selectivity of 30 and 24, which are all among the highest level of solid CO2 absorbents developed thus far. This work may inspire new sparks on rational disposal of tar-like byproducts from pyrolysis of organic solid waste and also encourage future utilization of them for advanced materials for innovative environmental applications, including CO2 capture and storage and gaseous pollution control, among others.