Allometric growth patterns of functional morphological variables that reflect resource use in fish were correlated with the diet of Diplodus capensis in southern Angola. A total of 114 individuals (76–336 mm FL) were collected for stomach content analysis and a further 35 individuals (80–320 mm FL) for morphometric analysis. The major dietary components belonged to the groups Chlorophyta, Cirripedia, Rhodophyta and Bivalvia, and made up 71.7% of the diet in terms of a computed ranking index. There was a clear ontogenetic dietary shift with small fish feeding predominantly on algae and large fish on barnacles and mussels. This increasingly durophagous diet corresponded with a positive allometric growth pattern in the gape dimensions (width, height and surface area), adductor mandibulae muscle process and incisor width. Conversely, the relative gut length followed an isometric growth pattern, suggesting a decreased absorptive capacity. This isometric growth pattern is attributed to the better digestibility of the durophagous prey items when compared with algae and further corroborates a link between the morphology and an ontogenetic dietary shift observed in this species.