The mechanism of doping carbon nanotubes (CNTs) with a salt solution was investigated using the density functional theory. We propose that the anion-CNT complex is a key component in doping CNTs. Although the cations play an important role in ionizing CNTs as an intermediate precursor, the ionized CNTs are neutralized further by forming a stable anion-CNT complex as a final reactant. The anion-CNT bond has a strong ionic bonding character and clearly shows p-type behavior by shifting the Fermi level toward the valence band. The midgap state is introduced by the strong binding of carbon and anion atoms. These localized charged anion sites are highly hygroscopic and induce the adsorption of water molecules. This behavior provides a new possibility for using anion-functionalized CNTs as humidity sensors.