A case of atraumatic rupture of the spleen originally presenting as ruptured sigmoid diverticulitis is described. The underlying disease was found to be myelofibrosis. Although progressive splenomegaly is always associated with this disease, spontaneous rupture of this organ is not reported in the literature. We also reviewed the literature in regard to unusual presentation of splenic rupture and we could not find any case simulating acute sigmoid diverticulitis. It is suggested that awareness and familiarity with this disease may enable the surgeon to suspect this diagnosis when dealing with an unusual acute abdominal emergency.