Pembrolizumab (Keytruda®) is an anti-programmed cell death 1-specific monoclonal antibody that has become the standard second-line chemotherapy for unresectable advanced microsatellite instability-high colorectal cancer. Several immune-related adverse events (irAEs), particularly endocrinopathy, are linked to the administration of pembrolizumab. We report here a case of pembrolizumab-induced isolated adrenocorticotropic hormone deficiency in a patient with metastatic colon cancer. A 65-year-old woman visited our hospital for complaints of fatigue with a recent history of primary resection of cecal mucinous cancer and hepatectomy for liver metastasis 3 years ago. Peritoneal dissemination was detected 2 years after surgery. Several chemotherapeutic regimens of cytotoxic and molecular targeted drugs were administered; however, the metastases progressed gradually. Pembrolizumab monotherapy was started because of resistance to treatment. After 2 cycles of pembrolizumab, the patient was severely fatigued. Laboratory data demonstrated that the cortisol level was extremely low. All the other values were within the normal range. Magnetic resonance imaging indicated no mass in the pituitary gland. From multiple tolerance tests, we diagnosed isolated adrenocorticotropic hormone deficiency caused by pembrolizumab. The patient's symptoms improved promptly with cortisol treatment. An abdominal contrast-enhanced computed tomography scan after 5 cycles of pembrolizumab demonstrated that the size of the peritoneal dissemination remained unchanged. However, her serum level of carcinoembryonic antigen had decreased to normal levels. Endocrine disorders are very rarely seen as irAEs. Careful laboratory data follow-up is required to inhibit the progression of severe endocrine disorders.