Seawater batteries (SWBs) represent a promising energy storage solution because they use abundant, low-cost Na ions from seawater. However, the power performance of SWBs is limited by the sluggish kinetics of the oxygen evolution and reduction reactions (OER/ORR) at seawater cathodes. In the present study, to address these limitations, we explored the use of manganese dioxide (MnO2) as a bifunctional electrocatalyst with pseudocapacitive behavior to enhance OER/ORR catalytic activity and power performance. MnO2 nanoparticles were synthesized via a straightforward precipitation method, resulting in a material with an amorphous structure, which is conducive to improved capacitive performance. Electrochemical characterization revealed that the SWBs with MnO2 exhibited significantly enhanced power output and cycling stability relative to the cell without the catalyst. These improvements are attributed to the hybridized effects of pseudocapacitance and catalytic activity, which accelerate charge storage and release. Our findings suggest that MnO2 is a promising material for enhancing the performance of SWBs, paving the way for developing next-generation energy storage systems with superior efficiency and stability.