to prevent their unit from freezing, landers, rovers and modules present on the Moon's surface require heat and electrical power during the lunar night. Fuel cells present a promising candidate for this application because of their high energy density and the possibility to exploit the excess heat they generate. This paper presents the mission requirements and the corresponding design of the Regenerative Fuel Cell System (RFCS) to tie in with the European Large Logistics Lander (EL3). The European Space Agency (ESA) provided the initial requirements for the project. Thales Alenia Space Italia (TASI), the Centre for Research and Technology-Hellas (CERTH) and the Foundation for Research and Technology-Hellas (FORTH), performs the system design during the first part of the development activities to bring it to TRL5. Additionally, PEAK contributed to the preliminary gas tanks design. The ensuing discussion encompasses the concept of operation, which involves alternating between electrolysis and fuel cell, as well as the transition between operational modes. It also addresses system architecture and technologies tradeoffs, elucidating the advantages and disadvantages of the available options for a lunar Regenerative Fuel Cell System (RFCS). The final design is then presented, discussing the expected performances.