We developed the synthesis of one-dimensional polythiophenes inside single-wall carbon nanotubes (SWCNTs). Direct vapor-phase reaction with dibromobithiophenes provided longconjugated polythiophenes through dehalogenative polymerization in the cavities of SWCNTs. Tethering of the large number of thiophene units was confirmed by optical absorption measurement and high-resolution transmission electron microscopic (HR-TEM) observation. Raman spectroscopy indicated that restricted one-dimensional inner space of SWCNTs enables s-trans coplanar conformation of thiophene moieties. We also revealed a weak p-doping of metallic SWCNTs selectively occurred by bromine molecules, which were generated during the synthesis of polythiophenes.