Aquaculture finfish production based on floating cage technology has raised increasing concerns on the genetic integrity of natural populations. Accidental mass escapes can induce the loss of genetic diversity in wild populations by increasing genetic drift and inbreeding. Farm escapees likely represent an important issue in the gilthead sea bream, which accounted for 76.4% of total escapes recorded in Europe during a three-year survey. Here, we investigated patterns of genetic variation in farmed and wild populations of gilthead sea bream from the Western Mediterranean, a region that faced a long-history of gilthead sea bream farming. We focused our attention on the role that genetic drift may play in shaping these patterns. Results based on microsatellite markers matched those observed in previous studies. Farmed populations showed lower levels of genetic diversity than wild populations and were genetically divergent from their wild counterparts. Overall, farmed populations showed the smallest population effective size and increased levels of relatedness compared to wild populations. The small broodstock size coupled with breeding practices that may favor the variance in individual reproductive success likely boosted genetic drift. This factor appeared to be a major driver of the genetic patterns observed in the gilthead sea bream populations analysed in the present study. These results further stress the importance of recommendations aimed at maintaining broodstock sizes as large as possible and equal sex-ratios among breeders, as well as avoiding unequal contributions among parents