In this case, a mass located subcutaneously in the perineal subvulvar region of a six-year-old Simmental cow was examined clinically, pathomorphologically and immunohistochemically. Macroscopically, the solitary, whitish yellow-tumor mass was 19x15x6 cm in size, weighed 1610 grams, and had a lobular structure with a few small cystic formations in the section. Histopathological examination revealed that the tumor parenchyma had smooth muscle-like cells with abundant cytoplasm, pleomorphic cells with blunt-ended or cigar-shaped nuclei, anisocytosis, anisokaryosis and karyomegaly. Immunohistochemically, strong positive expression for ??-SMA, vimentin, Ki67 and slight positive for desmin were found, while immulolabeling for pancytokeratin (AE1/AE3), S-100, CD31 and CD34 were negative. In conclusion, on the basis of these findings, the tumor was diagnosed as leiomyosarcoma.