Bisphenols (BPs) are chemical compounds largely used in the plastic industry and in several manufacturing processes. Their presence has been documented in several environmental and biological matrices. Many of them have been found to act as endocrine disruptors, causing negative effects on animals and humans. Bisphenol A is the precursor of all bisphenols, and given its documented toxicity, it has been banned and substituted by several analogues in different processes. Among them, Bisphenol S has been used worldwide, and its presence is reported in several matrices. Nevertheless, these alternative bisphenols do not seem to be safer than their congener. Given these premises, it is of great concern to monitor BPS presence in bivalve mollusks, which can be used as sentinels of environmental contamination. BPs are generally quantified using chromatographic techniques interfaced with high-resolution spectrometers. This study aims to identify a simple and reproducible method able to determine the BPS presence in mussels and to quantify it. For this purpose, a competitive ELISA method was validated and applied to the determination of BPS in mussels. The validated method was applied to the determination of BPS in mussels harvested along Sardinian coasts in Italy, and a mean concentration of 10 ng/g ww of BPS was found. The results of the present study are of great concern from a One Health perspective, given that they reflect the healthiness of the environment and of mussels collected in Sardinia, where over 13,000 tons of mussels are produced every year.