The shape (derived from landmark-based geometric morphometrics), condition (Fulton index) and diet (determined through gut content analysis) were described for the pike icefish Champsocephalus esox (Channichthyidae) from Última Esperanza sound, south-west Patagonia, Chile. Based on the length-weight relationship, females were heavier at length than males. Nevertheless, the Fulton index was similar between males and females. The morphospace of C. esox showed high intraspecific variability in the dorsoventral position of the tip of the snout, anus and the ventral insertion of the pectoral fin, as well as the anteroposterior position of the premaxilla, opercle and anus. This indicates the existence of phenotypic plasticity, leading to specimens with larger jaws and heads but shorter trunks, or specimens with shorter jaws and heads but larger trunks. This phenotypic plasticity was independent of size and sex. The feeding incidence was similar between sexes (34.1% and 47.2% for males and females, respectively). Diets consisted of only fish, small notothenioids of the genus Patagonotothen (P. tessellata, P. cornucola and P. sima), showing similarities between males and females. Finally, C. esox is the second notothenioid species, and the first outside of Antarctica, to display phenotypic plasticity in its body shape.