In primates, little has been reported about malignant mesenchymal uterine tumours. A case of a spontaneous metastasising uterine sarcoma in a 17-year-old rhesus monkey is presented. Clinically, transient abdominal pain, spasms, nausea, anaemia, a firm uterus and bloody vaginal discharge were noted. In a diagnostic laparoscopy, both massive adhesions in the lesser pelvis and 10 ml of ascites fluid were detected. In necropsy, in addition to peritonitis with massive adhesions, a cauliflower-shaped, irregular, tough, greyish-white uterine tumour was seen. Two cherry-sized tumour metastases were noticed in the greater omentum. In histology, both in the uterus and the metastases, a sarcoma with a low amount of connective tissue and well-differentiated cell nuclei was identified.