<p>The insular golden lancehead <i>Bothrops insularis</i> presents several morphological features that diverge between sexes and closely related species such as body size, stoutness, and head traits. These differences are often attributed to specificities in reproductive or ecological requirements and frequently reflect microevolutionary patterns. In this study, we evaluated sexual dimorphism and ontogenetic allometry in <i>B. insularis</i> and compared head shape with two mainland populations of <i>B. jararaca</i> using linear and geometric morphometrics to understand the patterns involved in morphological divergence. Females were larger than males for almost all body parameters analyzed, except for tail traits, and also had a larger and wider head. We found no difference in model slopes for body and head shape ontogenetic trajectories, indicating that both sexes shared common trajectories in postnatal development. Interspecific comparisons revealed marked differences in males’ head shape and ontogenetic trajectories. The head of <i>B. insularis</i> was phenotypically closer to the highland <i>B. jararaca</i> population which is in accordance with the phylogenetic affinity of <i>B. insularis</i> with this population. On the other hand, <i>B. insularis</i> showed a snout size similar to that of the coastal population. The resemblance in snout shape to the coastal population may represent an evolutionary divergence from a shared ancestor with the highland population as a consequence of island isolation and diet consisting of birds.</p>