Herein, we aimed to present a child with extremely severe hypertriglyceridemia (ESHTG) secondary to diabetic ketoacidosis concomitant with type IX glycogen storage disease (GSD). Extremely severe hypertriglyceridemia (10 700 mg/dL) was detected through the apparent lipemic appearance of the sampled blood in a 17 year-old male patient with severe diabetic ketoacidosis. In spite of insulin infusion, the patient's clinical condition deteriorated to acute pancreatitis. Single sessions of therapeutic plasma exchange (TPE) along with insulin treatment have successfully intercepted the progression of the state of acute pancreatitis. The patient was also diagnosed with type IX GSD on the basis of the genetic analyses performed for the potential underlying metabolic diseases. In conclusion, underlying metabolic diseases, such as glycogen storage disease, should be investigated in patients with diabetic ketoacidosis accompanied by severe hypertriglyceridemia. If ESHTG does not relieve despite insulin infusion, and/or acute pancreatitis occurs as a complication, TPE should be kept in mind.