ABSTRACT. Leiomyosarcoma in soft tissue has only been reported in humans or animals. We performed pathological examination of a 3-years-old male squirrel monkey which had a large mass in the right axilla. The mass in the right axilla was surgically resected but it soon reoccurred. This tumor was diagnosed as a leiomyosarcoma based on histopathology and immunohistochemistry using the expression of -smooth muscle actin and desmin. Leiomyosarcoma in squirrel monkey has been reported in one case, there was no reoccurrence or metastasis. Most of the reported tumors in nonhuman primates have occurred in aged monkeys. This case was a rare tumor considering the young age of the animal.