Diabetes mellitus type 2 or Type 2 Diabetes (formerly called non-insulin-dependent diabetes mellitus (NIDDM), or adult-onset diabetes) is a metabolic disorder that is primarily characterized by insulin resistance, relative insulin deficiency and hyperglycemia. It is often managed by engaging in exercise and modifying one's diet. It is rapidly increasing in the developed world, and there is some evidence that this pattern will be followed in much of the rest of the world in coming years. The CDC has characterized the increase as an epidemic. In addition, whereas this disease used to be seen primarily in adults over age 40, in contrast to Diabetes mellitus type 1, it is now increasingly seen in children and adolescents, an increase thought to be linked to rising rates of obesity in this age group, although it remains a minority of cases.

Unlike Type 1 diabetes, there is little tendency toward ketoacidosis in Type 2 diabetes, though it is not unknown. One effect that can occur is nonketonic hyperglycemia which also is quite dangerous, though it must be treated very differently. Complex and multifactorial metabolic changes very often lead to damage and function impairment of many organs, most importantly the cardiovascular system in both types. This leads to substantially increased morbidity and mortality in both Type 1 and Type 2 patients, but the two have quite different origins and treatments despite the similarity in complications.