Abstract. Background: Treatment adherence is associated with a number of personal characteristics (e.g., self-esteem, self-efficacy, locus of control) and illness perceptions. However, there is a lack of studies for Type 2 Diabetes Mellitus (T2DM). Aims: The aim of this study was to compare adherence behavior of British and Greek people with T2DM and examine the association between personal characteristics, illness perceptions, and adherence. Method: This was a cross-cultural, cross-sectional study. Five hundred eighty participants completed questionnaires, 208 males (35.9%) and 372 females (64.1%) with a mean age of 49 years. Regression analyses and t-tests were used. Results: Personal characteristics and illness perceptions predicted adherence and British patients reported better exercise adherence while Greek patients reported better diet and medication adherence; both groups reported suboptimal adherence. Self-efficacy, Health Locus of Control (HLoC), and illness perceptions were the main adherence determinants. Limitations: The use of self-reports and the convenience sampling method are the main limitations of this study. Conclusion: The findings address a significant gap in research and can inform future adherence-enhancing interventions to promote the well-being of people with T2DM.