Radioactive cesium, particularly Cs-137, poses a long-term environmental and health risk owing to its long half-life and high solubility and mobility in aquatic environments. To enable efficient removal and real-time monitoring, in this study, we developed a closed-loop adsorption–detection system employing Prussian blue (PB) as a selective adsorbent. The system consists of two sequential adsorption modules housed in 1 L Marinelli beakers filled with PB-alginate beads. A NaI(Tl) detector (1st module) and a LaBr3 detector (2nd module) were installed to enable continuous in situ gamma spectrometric monitoring. The adsorption performance of PB-alginate beads was initially evaluated using nonradioactive and radioactive Cs solutions. Nonradioactive tests using stable Cs solutions (5 and 50 ppm) demonstrated rapid adsorption, achieving removal efficiencies of 81 and 41% within 10 min, respectively. The distribution coefficient (Kd) analysis revealed a higher Kd at a low concentration (4.26 mL/g at 5 ppm) than at high concentration (0.695 mL/g at 50 ppm), suggesting that adsorption efficiency decreased under a high-concentration conditions because of the limited availability of adsorption sites. Radioactive Cs-137 tests validated the system’s real-time monitoring capability, achieving an overall removal efficiency of approximately 90%. These results indicate that the PB-alginate bead-based closed-loop system enables effective Cs-137 adsorption and real-time quantification, offering a promising platform for radiological emergency response and environmental remediation. By integrating PB-alginate bead-based adsorption with NaI(Tl) and LaBr3 detectors, the proposed system enables the continuous real-time monitoring of radioactive cesium in aqueous environments, demonstrating its applicability as a sensor-based monitoring system.