We report the first case of hypercalcemia-induced acute pancreatitis caused by a functioning parathyroid cyst in a 67-year-old man. Laboratory investigation revealed increased serum amylase and lipase, increased serum ionized calcium and parathyroid hormone (PTH) levels, and decreased serum phosphate, indicating pancreatitis and primary hyperparathyroidism (PHPT). Abdominal computed tomography (CT) revealed mild swelling of the pancreatic head with peri-pancreatic fat infiltration and fluid collection around the pancreatic tail. Ultrasonography and CT of the neck showed a cystic lesion at the inferior portion of the left thyroid gland, suggesting a parathyroid cyst. There was no evidence of parathyroid adenoma by 99mTc sestamibi scintigraphy. PHPT caused by a functioning parathyroid cyst was suspected. The patient underwent surgical resection of the functioning parathyroid cyst owing to his prolonged hypercalcemia. At 3 weeks after the operation, his serum levels of PTH, total calcium, ionized calcium, inorganic phosphate, amylase, and lipase were normalized. At the follow-up examinations, he has remained asymptomatic.