jats:titleSummary</jats:title>jats:pUndifferentiated sarcomas, more correctly known as undifferentiated pleomorphic sarcomas (UPS), are a form of soft tissue sarcoma. They appear to be uncommonly diagnosed in veterinary species but represent the second most common form of soft tissue sarcoma in human adults. There is an evolving nomenclature around these tumours, with older reports referring to ‘malignant fibrous histiocytoma’, but these have been reclassified into several different tumour types, including the UPS. This makes review of the literature challenging, as many tumours previously referred to as malignant fibrous histiocytomas have been reclassified and are not the same as the UPS. jats:italicEn bloc</jats:italic> surgical excision is the mainstay of treatment in all species, although adjunctive radiotherapy and perioperative chemotherapy improve the prognosis in humans, and should be considered in horses diagnosed with this tumour type. Metastasis is uncommon in all species but can be devastating when it occurs.</jats:p>