Localized surface-plasmon resonance affects the optical absorption and scattering of nanosized materials. The intensities and peak energies of the surface plasmons strongly depend on the carrier density; thus, the optical absorption peaks originating from the surface-plasmon resonance can be manipulated by the density of injected carriers. In single-wall carbon nanotubes (SWCNTs), the correct identification of surface-plasmon resonance modes is of great interest due to their emerging plasmonic and optoelectronic applications. Here, we demonstrate that high-carrier injection by electric double layers can induce a transverse surface-plasmon peak in aggregated, electricity-selected SWCNTs. In contrast to the well-discussed surface-plasmon resonance mode, whose polarization is parallel to the axis and whose resonance frequency is located in the THz region, our identified mode, which was normal to the axis, was located in the near-infrared range. In addition, our mode's peak position and intensities were tunable by carrier injections, indicating a route to control plasmonic optical processes by electric double-layer carrier injections using ionic liquid.