<p>The aim of this study is to propose a novel method for the synergetic seismic retrofitting and energy upgrade of existing masonry buildings based on multi-performance and sustainable materials. The method is based on the use of a novel type of flax-fiber-reinforced, cement-free concrete that uses clay, produced by earth excavated during construction, as a natural replacement of cement. The addition of natural fibers produced by the plant flax increases the strength of the cement-free concrete, which is characterized by substantially lower thermal conductivity and carbon footprint compared to conventional concrete. The material is integrated as an infill in a novel composite wall configuration that manifests high seismic and energy performance, as demonstrated in a large-scale experimental campaign at ETH Zurich. The obtained results show that the novel cement-free material and the novel mechanical wall configuration combine high seismic and energy performance with low environmental impact, thus facilitating the multi-performance rehabilitation of existing buildings.</p>