The chemical doping of single-walled carbon nanotubes (SWCNTs) using electron donor and acceptor molecules is a crucial step for controlling the frontier orbital energy gap of SWCNTs. This means that SWCNTs are a promising material for electronics applications such as thermoelectric conversion and thin-film transistors, due to their flexibility, stability and light weight. Among all chemical dopants, 2 organic dopants containing hetero atoms are important materials as they permit fine controllability of the doping level; they also provide stability due to the flexibility of their structural design. In this study, we found that pyridine-boryl (py-boryl) radicals serve as efficient electron-doping reagents for SWCNTs, in which the doping mechanism comprises electron transfer from the py-boryl radical to the SWCNT. The formation of a stable py-boryl cation is essential for efficient doping; the captodative effect of the py-boryl cation is important to this process.