Internal functionalization of single-walled nanotubes is an attractive, yet difficult challenge in nanotube materials chemistry. Here we report single-walled metal oxide nanotubes with covalently bonded primary amine moieties on their inner wall, synthesized through a one-step approach. Conclusive molecular-level structural information on the amine-functionalized nanotubes is obtained through multiple solid-state techniques. The amine-functionalized nanotubes maintain a high carbon dioxide adsorption capacity while significantly suppressing the adsorption of methane and nitrogen, thereby leading to a large enhancement in adsorption selectivity over unfunctionalized nanotubes (up to four-fold for carbon dioxide/methane and ten-fold for carbon dioxide/nitrogen). The successful synthesis of single-walled nanotubes with functional, covalently-bound organic moieties may open up possibilities for new nanotube-based applications that are currently inaccessible to carbon nanotubes and other related materials.