A vulvar leiomyosarcoma was diagnosed in an 8-year-old mixed-breed, sexually intact cat. The cat had a history of vulvar swelling, bleeding and stranguria. The mass was located at the ventral commissura of the vulva. The cut surface of the mass was lobulated and greyish-white, with areas of necrosis and haemorrhage. Histologically, it was a spindle-cell sarcoma consisting of interlacing bundles of very elongated neoplastic smooth muscle cells and variably dense collagenous matrix separated by individual cells or streams. It also contained areas of focal necrosis and haemorrhage. The mitotic index ranged from seven to 12 mitoses per 10 high-power fields. Immunohistochemically, the spindle-shaped neoplastic cells were strongly positive for smooth muscle actin (SMA) and vimentin, whereas no immunoreactivity was obtained for desmin. The vascular elements of the internal control tissue also exhibited strong SMA and vimentin immunoreactivity. Based on the morphological and immunohistochemical features, the tumour was classified as a leiomyosarcoma.