Sexual dimorphism is a common phenomenon in crustaceans, manifesting as morphological variations associated with intraspecific competition and reproduction. Ucides occidentalis, a key species in the mangrove ecosystems of the Eastern Pacific, exhibits sex-related morphological differences; however, its characterization remains limited. This study analyzes the morphological variability of the species’ carapace using geometric morphometrics to assess sexual dimorphism patterns. A total of 41 specimens (30 males and 11 females) were collected in the Gulf of Guayaquil, Ecuador. Carapace images were processed using Procrustes superimposition and principal component analysis to identify shape variations. Canonical discriminant analysis revealed significant sex differentiation (λ = 0.0412, X∑ = 79.75, p < 0.001), with males displaying a laterally expanded structure and higher morphological disparity, whereas females exhibited a rounded and conserved carapace morphology, optimized for reproduction. These findings highlight the influence of sexual selection and ecological pressures on the species' morphology. The application of geometric morphometrics provides a precise tool for characterizing sexual differences in crustaceans and can contribute to the development of conservation policies and sustainable management strategies for U. occidentalis.