Canine transmissible venereal tumor, also known as infectious sarcoma, venereal granuloma, transmissible lymphosarcoma, or sticker tumor, is a type of canine tumor that mainly affects the external genitalia, i.e., the vagina in bitches and the penis and prepuce in males. The tumor is usually nonmalignant, but metastasis to regional lymph nodes, spleen, liver, kidney, lung, brain, and skin has been reported in <6 % of cases. In this clinical case report, one 9-year-old, castrated male terrier dog was referred to Iranmehr Companion Animal Clinic in Mashhad, Iran, with multiple cutaneous cauliflower-shaped masses. Cytologic preparations of lesions revealed a population of round- to oval-shaped cells, with lightly basophilic cytoplasm that contained multiple distinct vacuoles. The clinical and histologic diagnosis was cutaneous transmissible venereal tumor in the progressing growth phase. Thoracic X-ray and abdominal ultrasonography did not show any internal metastasis. Large mass excisions under general anesthesia and further chemotherapy with vincristine sulfate induced clinical remission without recurrence during 9 months of follow-up. To the best of our knowledge, this is the first report of naturally occurring diffuse cutaneous transmissible venereal tumor in a mature male dog in Iran. It has been reported to be unique in that it is exclusively cutaneous without mucosal, subcutaneous, and other organ involvement.