<p>The study of cetacean reproductive physiology is important for their conservation; however, limited information is available on the reproductive characteristics of these species. The present study aims to characterize beluga whale (<i>Delphinapterus leucas</i>) sperm by analyzing their morphology, dimensions and shape, and proteome. Epididymal fluid samples (from each epididymis) were collected from a beluga whale housed in an aquarium that died (~ 60 years old). The results indicated that, in most cases, the sperm head has an ovoid shape with a slightly tapered anterior end. However, significant diversity in head morphology was observed, from elongated ovoid to shorter and broader, along with abnormalities in the midpiece-head connection (abaxial attachment). The morphometry of the sperm indicates a total length of 61.13 ± 2.25&#xa0;μm, which is significantly shorter (<i>p</i> &lt; 0.0001) than that of the other well-characterized cetaceans, such as the dolphin (<i>Tursiops truncatus</i>). The only parameter that did not differ significantly between the two species was midpiece length (<i>p</i> = 0.126). Additionally, a total of 1816 different proteins were identified by mass spectrometry analysis of sperm: 695 of epididymal origin, 418 proteins of sperm origin, and 698 being common to both, with 5 unclassified. Furthermore, a comparison of these proteins with those of other species of interest (human and dolphin) revealed similarities in sperm proteins among the three species. In conclusion, this study provides the first characterization of the morphometry and proteome of beluga whale sperm, opening the way for future research into new biomarkers, analysis of storage capacity, and potential applications in assisted reproductive technologies.</p>