Among the different sex determination modalities exhibited by fish species, polygenic sex determination (PSD) is supposedly unstable, with genetic and environmental components that may vary between populations exposed/adapted to different environments. European sea bass (Dicentrarchus labrax) is an interesting model, combining both a PSD system and a genetic subdivision into an Atlantic and a Mediterranean lineage, with genetic substructures within the Mediterranean Sea. We produced experimental progeny crosses (N = 927) from broodstock sampled in four wild populations (North Atlantic, NAT; Western Mediterranean, WEM; North-Eastern Mediterranean, NEM; South-Eastern Mediterranean, SEM). There were less females than males in the progeny, both in the global dataset (32.5%) and within each group of parental origin (from 25.1% for NEM to 39.0% for WEM), with significant variation among populations dams and sires. Sex, body weight (BW) and body length (BL) showed moderate heritability (0.52 ± 0.17, 0.46 ± 0.17, 0.34 ± 0.15, respectively). Sex was genetically correlated with BW and BL (rAsex/BW = 0.69 ± 0.12, rAsex/BL = 0.66 ± 0.13). The weighted genome-wide association study (wGWAS) performed both on the global dataset and within each parental origin, revealed a different genetic architecture of sex determination between Atlantic and Mediterranean populations, with a gradient of similarities from Western to Eastern Mediterranean populations, consistent with the hypothesis of a population-specific evolution of polygenic sex determination systems in different environments.