Conjugated polymers have attracted much attention for many years and have applications in various organic devices. Carbon nanotubes can be considered as all-carbon tube-shaped conjugated polymers containing only sp(2)-bonded atoms, which play an important role in nanotechnology and nanoelectronics. So far, no study has reported the realization of long pi-conjugated polymers as diameter-specified carbon nanotube segments. Herein, we report the first synthesis of a pi-conjugated polymeric segment (PSI) of armchair single-walled carbon nanotubes (SWCNTs). PSI is achieved by a rationally designed synthesis of a bifunctionalized cyclo-paraphenylene monomer, followed by inserting these ring-shaped units into the conjugated poly(para-phenylene) backbone. PSI was fully characterized by gel permeation chromatography (GPC) combined with NMR, FTIR, and Raman spectra. Possessing unique structural and physical properties, this long pi-extended polymer PSI can provide new insight for the development of bottom-up syntheses of uniform carbon nanotube segments and potential applications in electron- and hole-transport devices.