Clinical signs and necropsy findings were described in a four-year-old female rhesus macaque with diabetes mellitus, resulting from spontaneous amyloidosis. Weakness, polyphagia, polydipsia, and polyuria, which lasted for a few weeks, were determined from anamnesis and clinical investigations. Highly increased serum BUN, AST, ALP levels, glycosuria, and proteinuria was detected in the biochemical analysis. According to these clinical, haematological, and biochemical data, it was thought that the monkey was affected with diabetes mellitus. Despite all efforts to cure the animal, the patient died. Diffuse amyloidosis was detected in the kidney and pancreas at the necropsy and histopathological examinations.