In this study, we have shown how to prepare a ternary nanocomposite (Ag2S-NiMoO4-g-C3N4) consisting of graphitic carbon nitride (g-C3N4) nanosheets, silver sulfide (Ag2S) nanocrystals, and nickel molybdate (NiMoO4) nanorods and its sensing ability to detect quercetin, a flavonoid found in many fruits and vegetables. An Ag2S-NiMoO4-g-C3N4 nanocomposite-modified screen-printed electrode (SPE) exhibited remarkable sensing performance in a quercetin (Que) concentration range of 0.005 μM-20 μM with a low detection limit of 2.7 nM. Moreover, we have aimed at improving the selectivity and sensitivity of a sensor for detecting Que by optimizing the composition of Ag2S-NiMoO4-g-C3N4, the film thickness, and the electrolyte pH. The sensor's selectivity for Que was tested in the presence of potential interferents such as ascorbic acid, citric acid, fructose, glucose, lactose, maltose, mannose, sucrose, and tyrosine. The performance of the sensor was tested on a variety of food samples, including green apple, green tea, honey, and red onion skin.