A battery that uses seawater directly as part of it is called a seawater battery (SWB). Rechargeable SWBs have two distinctive merits: one is that it can directly use seawater, and the other is that it can be exposed to the marine environment. This chapter discusses the charge–discharge reaction of rechargeable SWBs, focusing on the cathode, which is the component most different from that of the sodium batteries. It covers each component material and technology development. The chapter reviews the history of SWBs and explains the operation principle and structure of the rechargeable SWBs as well as the reaction of the seawater cathode. The chapter then concerns with SWB constituent materials such as cathode materials, solid electrolytes, and anode and nonaqueous electrolytes. It also looks into the cell structures and briefly introduces the SWB commercialization cases.