The aim of this systematic review was to identify the reported barriers to regular exercise among adults either at high risk or already diagnosed with type 2 diabetes (T2D), because of the importance of exercise in the prevention of T2D. We searched the MEDLINE, Cinahl and PsycINFO databases. All potentially relevant articles were reviewed by two researchers, and 67 titles were found, of which 13 papers met inclusion criteria. Internal and external barriers to exercise were identified among adults either at high risk of T2D or already diagnosed. Internal barriers were factors which were influenced by the individual's own decision-making, and external barriers included factors which were outside of the individual's own control. It is important for counselling to identify the internal and external barriers to regular exercise. In this way, the content of counselling can be developed, and solutions to the barriers can be discussed and identified. Further research on the barriers to regular exercise is needed.