Enzyme-like nanocatalytic reactions developed for high signal amplification in biosensors are of limited use because of their low reaction rates and/or unwanted side reactions in aqueous electrolyte solutions containing dissolved O. Herein, we report a nitrosoreductase-like catalytic reaction, employing 4-nitroso-1-naphthol, Pd nanoparticles, and HN-BH, which affords a high reaction rate and minimal side reactions, enabling its use in ultrasensitive electrochemical biosensors. 4-Nitroso-1-naphthol was chosen after five hydroxy-nitro(so)arene compounds were compared in terms of high signal and low background levels. Importantly, the nanocatalytic reaction occurs without the self-hydrolysis and induction period observed in the nanocatalytic reduction of nitroarenes by NaBH. The high signal level results from (i) fast nanocatalytic 4-nitroso-1-naphthol reduction, (ii) fast electrochemical redox cycling, and (iii) the low influence of dissolved O. The low background level results from (i) slow direct reaction between 4-nitroso-1-naphthol and HN-BH, (ii) slow electrode-mediated reaction between 4-nitroso-1-naphthol and HN-BH, and (iii) slow electrooxidation of HN-BH at electrode. When applied to the detection of parathyroid hormone, the detection limit of the newly developed biosensor was ∼0.3 pg/mL. The nitrosoreductase-like nanocatalytic reaction is highly promising for ultrasensitive and stable biosensing.