Gelsolin amyloidosis is a dominantly inherited, incurable type of amyloidosis. A single point mutation in the gelsolin gene (G654A is most common) results in the loss of a Ca2+ binding site in the second gelsolin domain. Consequently, this domain partly unfolds and exposes an otherwise buried furin cleavage site at the surface. During secretion of mutant plasma gelsolin consecutive cleavage by furin and MT1-MMP results in the production of 8 and 5 kDa amyloidogenic peptides. Nanobodies that are able to (partly) inhibit furin or MT1-MMP proteolysis have previously been reported. In this study, the nanobodies have been combined into a single bispecific format able to simultaneously shield mutant plasma gelsolin from intracellular furin and extracellular MT1-MMP activity. We report the successful in vivo expression of this bispecific nanobody following adeno-associated virus serotype 9 gene therapy in gelsolin amyloidosis mice. Using SPECT/CT and immunohistochemistry, a reduction in gelsolin amyloid burden was detected which translated into improved muscle contractile properties. We conclude that a nanobody-based gene therapy using adeno-associated viruses shows great potential as a novel strategy in gelsolin amyloidosis and potentially other amyloid diseases.