This case report presents a one-year old pet female smooth-coated guinea pig with a diagnosis of leiomyosarcoma of the left ovary with locally aggressive behaviour. The animal had been showing lethargy and alopetic dermatosis for a few weeks. The mass in the area of the left ovary was suspected based on palpation, x-ray, ultrasonography of the abdominal cavity and the presence of the mass in the abdomen was confirmed by probatory laparotomy. Since the lesion was inoperable due to its size and infiltrative growth, the animal was euthanized. Leiomyosarcoma of the ovary with infiltrative growth into spleen and pancreas was diagnosed based on histopathological examination and the diagnosis was proved by immunohistochemical staining for the presence of smooth muscle actin. This is the first described case of this type of tumour in young intact female guinea pig. The case report could contribute to better understanding and knowledge of the occurrence of neoplastic changes of the ovary in guinea pigs especially for better treatment management and prevention of reproductive tract diseases.