In wild ballan wrasse Labrus bergylta, mass–length relationships were not different between genders, and positive allometry was found in the mixed‐gender population. Male‐biased sexual size dimorphism was significant and the most effective morphometric method for sexing L. bergylta outside of the species spawning window used body mass (MB in g), total body length (LT in mm) and Fulton's condition factor (K) as discriminant variables to predict gender with 91% accuracy. The discriminant score (SD) of a specimen can be calculated as SD = 0·01 MB − 0·016 LT − 3·835 K + 6·252 to predict its gender as female or male if SD is < 1·459 or SD is > 1·504, respectively. There was a potential trend towards earlier sexual inversion compared to previous studies at comparable latitudes. Sex change is a phenotypically plastic trait under social control in haremic fishes and should be monitored in increasingly exploited L. bergylta.