Onion being a semi-perishable horticultural bulb crop gets subjected to deterioration during the storage, which ultimately leads to huge monetary losses every year. Specific detection tools for onion spoilage will be an asset in relieving losses during post-harvest storage. In this context silver based yellow colored colloidal nanoparticle (AgY-NPs) solution was synthesized and evaluated as a visual sensor for the organo-sulfur compound released during the spoilage of onions. The visual changes during the spoilage was monitored for ten days wherein the yellow color of the AgY colloidal solution changed to orange, pink and finally turned transparent. Simultaneous analysis carried out by UV–Vis spectroscopy and colorimeteric analysis corroborates our investigation of AgY-NPs as a visual sensor displaying selectivity and specificity for volatile sulphur compound responsible for spoilage of onions. Furthermore, analysis of AgY-NPs in presence of healthy onions showed distinct visual results which were complemented by probable mechanism. Besides this the structural changes in AgY-NPs due to incorporation of sulphur compounds were supported by Powder X-ray diffraction (PXRD), Raman Analysis, Fourier Transform-Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) which clearly indicated the capacity of these AgY-NPs as a powerful detection tool for onion spoilage.