The anti programmed cell death-1 (PD-1) antibodies pembrolizumab and nivolumab have been recently licensed by the Food and Drug Administration for the treatment of advanced melanoma. Immune checkpoint inhibitors such as these can induce endocrine adverse events but autoimmune diabetes has not been described to date. However, there is a strong preclinical rationale that supports this autoimmune toxicity. We describe for the first time the case of an adult patient who developed autoimmune diabetes likely as a consequence of PD-1 inhibition with pembrolizumab. The presence of high serum titres of anti-glutamic acid decarboxylase antibodies together with a suggestive clinical presentation, age of the patient and preclinical data strongly support an autoimmune aetiology of the diabetes. Moreover, the patient was found to have a well-known high-risk human leucocyte antigen type for the development of type 1 diabetes in children, so the PD-1 inhibition is very likely to have triggered the autoimmune phenomenon. Our case suggests that insulin-dependent diabetes might be a rare but important anti-PD-1 immune-related adverse event.