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We proposed a novel electrochemical system for hydrogen production and power generation based on a fuel cell/battery system. The reactions (hydrogen evolution and oxidation) in the system occur at solid/liquid and solid/gas two-phase boundaries. From a theoretical analysis of the energy efficiency, we found that this novel electrochemical reactor has high energy efficiency for both hydrogen production and power generation. Following the theoretical analysis, electrochemical charge/discharge performance characteristics were studied at various current densities by fabricating an experimental cell, which was composed of negative and positive electrodes with a 1:3 capacity ratio. From the experimental results, we found that hydrogen gas was produced in a linear manner during the charge process and it was completely consumed during the discharge process. The energy conversion efficiency of the hydrogen production and the round-trip efficiency of hydrogen production and power generation at a current density of 37.0 A/m2 were 98.3% and 79.6%, respectively. These results indicated that the present system is highly efficient compared with the conventional water electrolysis and/or power generation systems.