Biometrics have been widely employed to predict sex when clear dimorphism is lacking. In this study, we aimed to assess sexual dimorphism in the Mediterranean Shag (Phalacrocorax aristotelis desmarestii) and provide predictive functions for sex determination based on biometrics. A total of 102 individuals were employed, with 62 individuals used for model construction and 40 for effectiveness testing. Sex determination was accomplished through feather samples via CHD gene amplification for live birds and gonadal inspection for deceased birds. Eleven measurements were taken and analysed: bill depth, bill width, culmen length, bill length to feathers of side, wing length, 3rd primary length, tarsus length, tarsus depth, tarsus width, tail length, and body mass. We employed Generalized Linear Models (GLM) to construct the predictive equations. Sexual dimorphism was evident, with males displaying significantly larger bill depth, tarsus length, and wing length than females. The developed models, based on these variables, achieved accuracies ranging from 89.29% to 97.62% in sexing the birds. Determining the sex of birds enhances insights into behavioural and population dynamics of this vulnerable subspecies.