Acromegaly is a rare endocrine disease caused by hypersecretion of growth hormone, most commonly arising due to a pituitary adenoma. Diabetes mellitus is a common complication of acromegaly, occurring in approximately one-third of patients. The risk of diabetes mellitus in acromegaly is driven by increased exposure to growth hormone, which directly attenuates insulin signalling and stimulates lipolysis, leading to decreased glucose uptake in peripheral tissues. Acromegaly is a unique human model, where insulin resistance occurs independently of obesity and is paradoxically associated with a lean phenotype and reduced body adipose tissue mass. Diabetes mellitus in patients with acromegaly is associated with an increased risk of cardiovascular morbidity and mortality. Therefore, preventive measures and optimized treatment of diabetes mellitus are essential in these patients. However, specific recommendations for the management of diabetes mellitus secondary to acromegaly are lacking due to limited research on this subject. This Review explores the underlying mechanisms for diabetes mellitus in acromegaly and its effect on morbidity and mortality. We also discuss treatment modalities for diabetes mellitus that are suited for patients with acromegaly. Improved understanding of these issues will lead to better management of acromegaly and its associated metabolic complications.