Rechargeable seawater batteries (SWBs) using seawater as a catholyte have attracted extensive attention owing to the ocean's high theoretical energy density (3051 Wh L-1, 3145 Wh kg-1) and excellent thermal management. However, despite many improvements in materials and cell designs used in SWBs so far, there is a limit on the energy density in practical use, because of the lack of optimization of the cell structure. Herein, this work introduces a novel design by applying a rigid frame with an extended space called “prismatic-type.” Consequently, an energy density of 23 Wh (242 Wh L-1) is obtained by increasing the specific area of the unit cell and the capability of the anode active materials in the internal space. In addition, it enables the design of a discrete type that can improve the power density by increasing the surface area of the cathode current collectors. With the increased surface area, a peak power of 1162 mW is achieved for the discrete type compared to 727 mW for the integral type. These results suggest that these newly designed prismatic SWBs could contribute to practical applications in the near future.