Tracing the dynamical behavior of light-induced surface charges on nanostructures is pivotal for elucidating the fundamental processes governing catalysis and light-nanomaterial interactions. Extension to nanoscale surface charge dynamics, however, remained challenging due to the involved length and time scales. Here, we demonstrate direct observation of surface charge dynamics on dielectric nanoparticles under strong-field irradiation using time-resolved reaction nanoscopy. Our technique offers four-dimensional visualization of the spatiotemporal evolution of localized charges on spherical SiO2 nanoparticles, providing unprecedented resolution at the femtosecond-nanometer scale and single-nanoparticle level. Through semi-classical simulations, we reveal the distinct roles and characteristic timescales of diffusion and decay processes driving surface charge relaxation on SiO2 nanoparticles. Quantum dynamical simulations reveal bond-weakening in adsorbate molecules at the nanosurface, attributed to the presence of the surface charges. Unveiling nanoscale charge dynamics and their impact on surface molecular bonding highlights the potential for designing nanomaterials with tailored functionalities, propelling atomic-scale technologies and catalysis to new frontiers.