Carbon dioxides (CO2) emitted from large-scale coal-fired power stations or industrial manufacturing plants have to be properly captured to minimize environmental side effects. From results of ab initio calculations using plane waves [PAW-PBE] and localized atomic orbitals [ONIOM(wB97X-D/6-31G*:AM1)], we report strong CO2 adsorption on boron antisite (B-N) in boron-rich boron nitride nanotube (BNNT). We have identified two adsorption states: (1) A linear CO2 molecule is physically adsorbed on the B-N, showing electron donation from the CO2 lonepair states to the B-N double-acceptor state, and (2) the physisorbed CO2 undergoes a carboxylate-like structural distortion and C=O pi-bond breaking due to electron back-donation from B-N to CO2. The CO2 chemisorption energy on B-N is almost independent of tube diameter and, more importantly, higher than the standard free energy of gaseous CO2 at room temperature. This implies that boron-rich BNNT could capture CO2 effectively at ambient conditions.