Immune checkpoint inhibitors (ICIs) have been increasingly used in the treatment of various advanced cancers; however, therapy can be complicated by immune-related adverse events (irAEs). We present the case of a man in his 40s, with metastatic melanoma treated with nivolumab immunotherapy who developed ICI-induced diabetes mellitus (ICI-DM). Hyperglycaemia in the absence of ketoacidosis was incidentally noted when he presented to the emergency department for review of an urticarial rash. Further testing, including haemoglobin A1c and C-peptide level, confirmed his presentation was most consistent with ICI-DM and he was commenced on appropriate diabetes treatment. This report aims to detail an atypical presentation of ICI-DM and to highlight the importance of clinician awareness in identifying this irAE in patients receiving ICIs.