The growing need for structural, energetic and architectonic refurbishment of the existing structures led to the emergence of alternative retrofitting techniques. Among them, steel exoskeletons stand out for their non-invasive and time-efficient nature, especially in cases where the relocation of the building’s activities is not allowed. In this research, the optimal exoskeleton configuration, in terms of number of exoskeletons and their position around the building, along with the sizing of their constituent elements, has been obtained through an optimization process. The optimization tool is based on a Genetic Algorithm with the aim of weight minimization, including as constraints a maximum allowable inter-storey drift for preserving the elastic behavior of the existing building, and the structural safety of the exoskeleton members. Two exoskeleton typologies were selected as case studies, employed for the retrofitting of an existing RC moment-resisting frame building. Furthermore, Life Cycle Assessment analyses were conducted on the optimal configurations considering two materials: steels with high and low percentages of recycled materials. The results yielded interesting insights into the structural performance of both exoskeleton typologies, as well as into the environmental characteristics of both studied materials.