BACKGROUND An adult male owl monkey (Aotus nancymae) underwent a splenectomy. When the spleen was removed, a small, nodular mass slightly bulging over the splenic surface was noted. METHODS The mass was examined by light and transmission electron microscopy and by immunohistochemistry. RESULTS On light microscopy, the mass was well-circumscribed, non-encapsulated, and composed of haphazardly arranged smooth muscle bundles admixed with numerous small capillary-like structures containing blood. Immunohistochemical (IHC) staining revealed the tumor was strongly positive for smooth muscle actin yielding vascular smooth muscle bundles, and for Factor VIII, staining endothelial cells within the smooth muscle bundles. Transmission electron microscopy (TEM) showed a large portion of the cells to be atypical appearing smooth muscle and a few cells had structures resembling Weibel-Palade bodies indicating endothelial cells. CONCLUSIONS Based on cell morphology, by light and TEM, and IHC a final diagnosis of splenic angioleiomyoma was made. This is, to our knowledge, the first report of an angioleiomyoma in a non-human primate.