Addison's disease or primary adrenal insufficiency (hypoadrenocorticism) is an endocrine disease characterized by a deficiency in the production of the adrenal cortex hormones, particularly cortisol and aldosterone. Patients affected by Addison's disease develop anorexia, weight loss, hypoglycemia, fatigue, nausea, vomiting, skin, and mucosal pigmentation. Skin pigmentation is considered a commonly encountered manifestation of the disease; however, oral mucous membrane pigmentation is rarely noticed early enough for diagnosis. This case report demonstrates a case where oral pigmentation was an early diagnostic sign of Addison's disease.