This study presents a novel electrochemical sensor for the sensitive and selective detection of nitrite, based on gold nanoparticles (AuNPs) immobilized on biochar (BC). The BC/Au composite material was synthesized through a simple and cost-effective process and was characterized by various techniques to confirm its successful formation. The composite was then utilized to modify a fluorine-doped tin oxide (FTO) electrode, which served as the sensing platform. The sensor demonstrated remarkable sensitivity, selectivity, and stability towards nitrite detection, with a wide linear range and a low detection limit. The performance of the sensor was also evaluated using real water samples, showing good reproducibility and applicability for practical applications in environmental monitoring and food safety. This study provides an innovative approach for the development of electrochemical sensors based on BC/Au composites, contributing to the field of analytical chemistry.