The transmissible venereal tumor corresponds to the high frequency preputial neoplasm in dogs. This fact leads to the presumptive diagnosis, only motivated by the anatomical location and macroscopy without the aid of laboratory tests. However, such treatment may lead to therapeutic mistakes because other tumors (although less common ones) affect the preputial region, among them the papilloma, carcinoma, mast cell tumor and other skin tumors such as hemangiosarcoma. Therefore, the present study aimed at describing a case of hemangiosarcoma in a dog's prepuce as a way of discriminating the ordinary transmissible venereal tumor. A three-year-old Weimaraner dog was attended. His owner reported a tumor near the penis. The animal was physically examined and laboratory tests were asked. These exams corresponded to tumor cytology and serology for leishmaniasis (by the indirect methods of immunofluorescence and enzyme-linked immuno sorbent assay. However, the dog went through euthanasia and was sent for necropsy with the samples left in 10% formaldehyde solution and sent for histopathology. Clinically, there was a neoformation of 12.5 cm in diameter, located in the cranial portion of the prepuce that showed dermal and subcutaneous coverage with firm consistency, ulcerated surface with hemorrhagic exudation, and looking similar to cauliflower and friable. Other clinical observations were equivalent to onychogryphosis, hyperkeratosis of the nasal planum and cutaneous desquamation of the pinnae. According to the signs observed the clinical suspicion corresponded to the preputial tumor (with emphasis on the transmissible venereal tumor due to its anatomic position and its macroscopic characteristics) associated with canine visceral leishmaniasis. The cytology suggested the incidence of malignant neoplasm of mesenchymal origin, but it was not detected characteristic cells of transmissible venereal tumor. The serology for leishmaniosis was reagent in the two methods used and the owner followed the Brazilian Health Department recommendations choosing for euthanasia of the animal. The necropsy showed that the area had preputial neoformation with the cutting ranging from white to dark red and lymphadenopathy of inguinal lymph nodes, mesenteric, aortic-lumbar and hepatic. The histopathologic diagnosis of the preputial tumor was a hemangiosarcoma and there was reactional hyperplasia in lymph nodes and cystic degeneration. For all canine preputial tumors it is necessary a differential diagnosis between transmissible venereal tumor and other neoplasms like hemangiosarcoma because there are meaningful differences in treatment and prognostic.