Endometriosis is a relatively common condition in women and some populations of adult female rhesus macaques. However, endometriosis with extensive smooth muscle proliferation, as occurs in endomyometrioma and uterus-like mass (ULM), is rare in women. This report describes a case of endometriosis with extensive smooth muscle metaplasia resembling multiple ULM in a 20-y-old female rhesus macaque. During a protocol-related procedure, a large, smooth, globoid, freely moveable mass was palpated in the midabdomen. Ultrasonography revealed a cystic structure from which dark brown fluid was aspirated. During exploratory laparotomy, an 8-cm spherical mass in the greater omentum and 3 additional masses (diameter, 2 to 5 cm) attached to the omentum were excised. Microscopic examination of the masses revealed numerous foci of ectopic endometrial glands and stroma frequently surrounded by bundles of smooth muscle and fibrous connective tissue. The gross and histologic lesions in this macaque bore many similarities to ULM in women. To our knowledge, this case represents the first report of endometriosis resembling a uteruslike mass in a NHP.