Isolated submandibular sialadenitis is extremely rare in the neonatal period. All reported cases had predisposing factors such as prematurity, dehydration, anatomic deformities, calculi, and long-term oro-gastric tube feeding. Here, we present a case of neonatal isolated submandibular sialadenitis without any known risk factors and who was diagnosed later with hyperthyroidism secondary to maternal Graves' disease. Both sialadenitis and hyperthyroidism resolved with appropriate treatment. While both conditions are rare and important in neonatal emergency care, the possibility of neonatal hyperthyroidism should be explored in cases with isolated sialadenitis if there is no other risk factor. Possible mechanisms for this uncommon association are discussed.