A 33-year-old Japanese man with a history of atopic dermatitis and asthma had never been diagnosed with any apparent glucose intolerance but had been aware of palpitations for >10 years. A 75g oral glucose tolerance test (OGTT) at his physical examination in March 2021 revealed fasting hyperglycemia and post-load hypoglycemia. An OGTT recheck was performed in May 2021 and was normal. We hypothesized that gluconeogenesis from the liver had caused his fasting hyperglycemia and performed abdominal echocardiography, which revealed a right adrenal tumor with abnormal catecholamine production. We diagnosed pheochromocytoma and performed a right adrenalectomy in September 2021. Postoperatively, the patient's palpitations disappeared and his laboratory findings normalized. Glucose intolerance is well known to occur before surgery in patients with pheochromocytoma, but it is extremely rare that hypoglycemia is indicated by a presurgery OGTT, as in our patient's case. Only three similar cases are reported to date, and in all three, hypoglycemia occurred ≥2 hr after loading, accompanied by excessive insulin secretion compared to the plasma glucose level. Our patient's case is the only one in which preload hyperglycemia was observed. Before his OGTT, he had run from the train station to the hospital, which was likely to be the cause of the preload hyperglycemia. We speculate that the stimulation of adrenergic β2 receptors may be involved in the enhancement of insulin secretion in patients with pheochromocytoma, but the mechanism is unknown. Further reports may clarify the mechanism of hypoglycemia induced by pheochromocytoma.