Abstract In order to know the morphological variations of Goyazana castelnaui, we conducted an ontogenetic study from the first crab stage to the adult stage using geometric morphometry. We studied 36 females, 43 males and 162 juveniles collected in the Pajeú River, Brazil. We photographed crabs’ carapace, pleon and right cheliped with they arranged parallel to the plane, afterwards landmarks and semi-landmarks were strategically distributed in the images. Through principal components analysis, we observed that the variation occurred mainly in the frontal and posterior region of the carapace. The pleon presented marked variations in the posterior and anterolateral region, while chelipeds presented greater variations at the base of the fixed finger. The canonical variation between sexes and between juveniles and adults (p<0.05) varied significantly. Correct allocations obtained by discriminant analysis (p<0.05) varied between 89.3% and 100.0%. The carapace shows a constant dimorphism along the ontogenetic trajectory, and both sexes reach similar centroid sizes (p>0.05). While the pleon and the cheliped, both sexes are similar when juvenile, diverging along the trajectory and reaching different centroid sizes (p<0.05). Females have a larger pleon and males a larger cheliped, corroborating the monophyletic theory of Brachyura.