Misfolding loss-of-function diseases are a huge burden for people and states. Primary hyperoxaluria type 1 (PH1) is a rare genetic disorder caused by mutations in the alanine:glyoxylate aminotransferase 1 (AGT) enzyme. The underlying molecular mechanisms causing PH1 are associated with protein misfolding (enhanced aggregation and mitochondrial mistargeting). The main therapeutic approach to increase patients lifespan and quality of life is a double transplantation of kidney and liver. Alternative treatments are currently under study, such as gene and enzyme replacement therapies and pharmacological chaperones as treatments, but other alternatives are necessary. In this work, we developed and characterize a novel biotechnological approach using six single domain nanobodies (NB-AGT-1 to -6) as potential therapeutics for PH1 misfolding. We show that NB-AGTs are very stable proteins and bind to pathogenic and non-pathogenic variants of AGT with extreme affinities (with Kd values from low nM to low pM). Structural studies showed that NB-AGTs bind to different epitopes of AGT with selectivity for different AGT variants. Experiments in cellular PH1 models showed that internalization of engineered NB-AGT-3 enhanced the specific activity of disease-associated variants and retargeted the protein to peroxisomes. Overall, we show that NBs are a novel and promising approach to treat PH1 as well as other loss-of-function misfolding diseases.