Recently, extraordinary attention has been given to the preparation of strong acid-doped polyaniline in distinct morphologies, such as nanofibers, nanotubes, and nanoflowers, while seeking proper and cheap energy polymeric materials. This work introduces the CO2-supported polymerization process as an advanced and novel polymerization technique to prepare rectangular cross-sectional self-assembled HCl-doped polyaniline nanotubes (HCl-PANNT) without the need to use any surfactants for the first time. Based on the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results, a rectangular cross-sectional HCl-PANNT with an average outer diameter and wall thickness of 110 and 35 nm, respectively, was prepared. Furthermore, X-ray powder diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) were employed to reveal the crystallinity and the chemical structure of the prepared polyaniline, respectively. Accordingly, the produced PANNT has a partial crystalline structure and emeraldine salt form. Moreover, the produced HCl-PANNT showed a high electrical conductivity of 1.5 S/cm.