Ovarian cyst is common incidental finding in humans and many animals and includes follicular cysts, cystic rete ovarii and mesonephric duct cysts. Ovarian cyst is often associated with reproductive disorders in humans and animals. We found accidentally bilateral cystic masses in ovaries in an African green monkey. Grossly, the left and right ovarian cystic masses were single unilocular cystic structures measuring 0.6 and 1.8 cm in diameter, respectively. Histologically, both cysts were thin-walled structures that arose from the center of the ovary and displaced ovarian tissue peripherally. The cysts were lined by a single layer of nonciliated low cuboidal epithelium. Immunohistochemically, epithelial cells in the cysts were positive for cytokeratin, and the stromal cells were positive for smooth muscle actin but negative for vimentin. These results suggest that these ovarian cysts in an African green monkey are cystic rete ovarii. To our knowledge, this is the first report of cystic rete ovarii in African green monkeys and may be of value in relation to research of the pathogenesis and treatment of ovarian cyst.