Seawater electrolysis, taking advantage of the huge seawater resource, holds great promise for sustainable hydrogen generation. Compared to conventional water electrolysis, seawater electrolysis is more challenging because of the more complex and corrosive electrolyte and competitive side reactions, which necessitates the development of highly efficient and stable electrocatalysts. In this study, a self-supporting, highly porous NiFe-PBA (Prussian-blue-analogue) electrocatalyst with a hierarchically hollow nanostructure is introduced, which exhibits impressive catalytic performance towards the oxygen evolution in alkaline seawater electrolytes. In NiFe-PBA, the synergistic interaction between Ni and Fe improves intrinsic conductivity for efficient electron transfer, enhances chemical stability in seawater, and boosts overall electrocatalytic activity. The direct use of self-supporting NiFe-PBA as an electrocatalyst avoids the energy-intensive and tedious pyrolysis procedure during the preparation process while making use of the tailored morphological, structural, and compositional benefits of PBA-based materials. By combining the NiFe-PBA catalyst with the NiMoN cathode, the constructed two-electrode electrolyzer achieved a high current density of 500 mA cm-2 at a low cell voltage of 1.782 V for overall electrolysis of alkaline seawater, demonstrating excellent durability for 100 hours. Our findings have important implications for the hydrogen economy and sustainable development through the development of robust and efficient PBA-based electrocatalysts for seawater electrolysis.