This case study describes a persistent Mü̈llerian duct syndrome (PMDS), a rare form of XY disorder of sex development (DSD), in a mature (>15 yr) beluga whale Delphinapterus leucas. The phenotypically and genetically male beluga whale had both Mü̈llerian (paramesonephric) and Wolffian (mesonephric) duct derivatives. A mild hydrometra was present. Gross and histological analyses indicated the testes were atrophied. Histopathologic findings in the testes also included fibrosis in addition to ductus deferens ductular dilation, epididymal ductular dilation, lymphoplasmacytic balanitis, uterine glandular atrophy, and endometrial fibrosis. DSDs are rarely reported in cetaceans, and PMDS has never been described in a cetacean.