Sexual dimorphisms represent a source of phenotypic variation and result from differences in how natural and sexual selection act on males and females within a species. Identifying the genetic basis of dimorphism can be challenging, especially once it is fixed within a species. However, studying polymorphisms, even when fixed within a population, can provide insights into the genetic basis of sexual dimorphisms. In this study, we investigate the genetic basis of a regionally isolated sexual dimorphism in the wings of Pieris napi adalvinda, a subspecies of P. napi found in northernmost Scandinavia, where females exhibit heavily melanized wings. By using a combination of male and female informative crosses, genomic sequencing of melanic outliers, and a population genomic analysis with a new reference genome for the melanic morph, we demonstrate that the female-limited morph adalvinda is caused by a single dominant allele at an autosomal locus upstream of the gene cortex. This novel finding adds to the growing body of literature that connects repeated mutations in and near the cortex gene to the regulation of butterfly wing melanization, providing insights into the evolution of sexual dimorphisms and the recruitment of genes into monomorphic or sex-limited forms. This study thus highlights the significance of cortex as a basis for a female-limited trait and lays the foundation for future comparative analyses of dimorphism genetic underpinnings.