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Summary The positive impact of the complementarity between renewable generation sources has been presented in several studies, but this concept has been neglected in many studies of expansion planning. This paper presents a new approach to introduce the complementarity between renewable energy resources and regions into the expansion planning for 100% renewable energy systems. A novel objective function is proposed to optimize the electricity mix and the water flow of hydropower reservoirs, considering the daily and yearly variations. A case study is presented with the objective of assessing the optimum mix (for the complementarity perspective) between solar, wind, hydro, and biomass power to ensure the expected monthly and hourly energy demand for Brazil in 2050. In the Brazilian case, when the complementarity between regions was considered, it was possible to ensure the electricity demand increase without new large reservoirs and with a high contribution from all renewable generation sources. A mix to ensure the daily and annual demand in 2050, which promotes the best complementarity, is presented, resulting in a share of 27%, 16%, 34%, and 23% for photovoltaic, wind, hydro, and biomass power, respectively.