Cesium-doped tungsten bronze Cs_xWO₃ (CWO) has potential applications in the field of energy-saving products due to its high-near-infrared (NIR) shielding ability. We for the first time demonstrate sensitive detection of small polaron transitions in the NIR for CWO nanoarchitectures on the basis of magnetic circular dichroism (MCD) spectroscopy. Two different CWO nanostructures are prepared by organic colloidal and solvothermal methods. Cube-like CWO nanoparticles obtained by the colloidal method exhibit a broad absorption peak at 1500 nm and a weak shoulder at 950 nm, which can be attributed to the localized surface plasmon resonance (LSPR) and small polaron transition, respectively. On the other hand, CWO nanoparticles synthesized by the solvothermal protocol show only a single LSPR peak. Simultaneous deconvolution of both absorption and MCD spectra allows us to discuss the detailed nature of the electronic excitations, and consequently, MCD distinctly unveils the small-polaron transition unresolved in the absorption. Hence, the MCD spectroscopic technique can be a significant optical-modulation method allowing for design of magneto-optically active materials in nanophotonics.