While conventional wisdom suggests the scattering centroid of a plasmonic nanoparticle reflects its geometric center, here we uncover the dependence of a scattering centroid of a single gold nanorod (AuNR) on its electron density when the geometric features (position and morphology) do not change at all. When periodically altering the electron density of a single AuNR during nonfaradaic charging and discharging processes, the optical centroid of the scattering dot in a series of dark-field images was found to reversibly shift back and forth by ∼0.4 nm, in pace with the sweeping potential. A Fourier-transform-based demodulation method was proposed to determine the centroid displacement as small as 0.1 nm, allowing for validating the generality of the observed phenomenon. The dependence of an optical centroid on the potential was attributed to the displacement of the electron density center as a result of inhomogeneous accumulation of injected electrons on the surface of a single AuNR. Not only does the present work shed light on studying the photon-electron interactions at sub-nanoparticle level, Fourier transform-based demodulation also provides a superior strategy for other fast and reversible processes such as electrochromic and photothermal conversions.