Abstract Cancer immunotherapies are changing the landscape of cancer care. Intratumoral talimogene iaherparepvec (T-VEC), an oncolytic viral vaccine, has been approved for treatment of unresectable melanoma with minimal toxicity. We describe the first case of a centenarian who developed autoimmune diabetes while on T-VEC immunotherapy. The patient’s high titer of glutamic acid decarboxylase 65 autoantibodies as well as insulin deficiency are consistent with autoimmune diabetes. Autoimmune diabetes has previously been seen following immune checkpoint inhibitor use; however, it has never been reported with T-VEC. This case highlights that autoimmune diabetes can be a rare but morbid complication of intratumoral T-VEC immunotherapy and can occur in the ultra-elderly.