This paper presents the gross and histopathologic findings of polyostotic fibrous dysplasia which is rarely seen in non-human primates. A 3-year-old female spider monkey (Ateles paniscus) was referred to the clinics of our faculty with the complaints of severe dyspnea, lethargy and cough and was diagnosed with dilated cardiomyopathy following the radiographic examination. However, the monkey died due to depression of respiration before treatment was done. Necropsy was performed on the request of the owner. Dilated cardiomyopathy and irregularly shaped tumoral masses with diameters of 0.5-1.0 cm in the right and left ribs were detected. Following the histopathological examination, the lesions were diagnosed as fibrous dysplasia.