This cross-sectional study employed functional near-infrared spectroscopy (fNIRS) to investigate the associations of moderate-to-vigorous physical activity (MVPA) and sedentary behavior (SB) with visuospatial working memory (VSWM) performance and task-related prefrontal cerebral hemodynamics in children. Seventy children aged 7–10 years wore accelerometers to assess MVPA and SB and completed a visuospatial 2-back task while simultaneously recording prefrontal cerebral hemodynamics. Bivariate analyses showed that higher MVPA was associated with lower SB, better VSWM performance (response accuracy and d-prime [d′]), and increased oxyhemoglobin (HbO) concentration in the right prefrontal cortex (PFC). SB was not significantly associated with any cognitive or hemodynamic outcomes. Both response accuracy and d′ were positively associated with right PFC HbO. Mediation analyses indicated that right PFC HbO partially mediated the relationships between MVPA and both VSWM outcomes, even after adjusting for sex, IQ, age, BMI, and SB. Sensitivity analyses confirmed that these indirect effects remained significant without covariate adjustment, although the direct effects became non-significant. These findings suggest that MVPA may enhance children’s VSWM through its influence on right PFC oxygenation, underscoring the role of cerebral hemodynamics in linking MVPA and VSWM during childhood development.