Plasmonic effect plays a significant role in many optoelectronic devices and enables various innovative applications. It has been widely studied in metallic materials, for example, Ag and Au, and later was expanded to transparent conductive oxides, etc. However, such plasmonic structures have limitations in many emerging optoelectronics including flexible optoelectronics, organic optoelectronics, and so on, due to their inorganic natures. In this manuscript, we discovered that the acid-modified highly conductive organic PEDOT:PSS film shows interesting plasmonic properties in the vis–NIR region and exhibits great potentials for activating surface plasmon polaritons. The dispersion curves of dielectric permittivity and optical constants of two modified PEDOT:PSS samples are obtained by inversion calculation of the spectroscopic ellipsometry data with the Drude–Lorentz dispersion model. The permittivity crossover wavelengths λC and the surface plasmon wavelengths λsp are found to be located squarely in the 650–900 nm range, which will enable future plasmonic device applications in the vis–NIR region. The activation of surface plasmon polaritons propagation mode of modified PEDOT:PSS is directly observed and confirmed by prism coupling experiments. In addition, further quantitative analysis revealed that our modified PEDOT:PSS samples have comparable abilities to generate, propagate, and confine surface plasmon polaritons as indium tin oxide (ITO). To the best of our knowledge, this is the first direct demonstration of an organic structure showing equivalent plasmonic properties to the inorganic ones. We believe it will open up much more possibilities for the optoelectronic devices, due to the flexibility, lightness, biological compatibility, and solution processability of the organic plasmonic materials.