Individuals managing diabetes are required to adhere to self-management behaviors to ensure the optimal regulation of their blood glucose levels. This study examined the psychological determinants underlying three important diabetes self-management behaviors (e.g., physical activity, diet, and blood glucose monitoring) using the reasoned action approach (RAA) and planning. A cross-sectional design was used, with participants (N = 273) completing measures of RAA constructs (e.g., experiential and instrumental attitude, descriptive and injunctive norm, and capacity and autonomy) and planning (e.g., action and control planning) at time 1 and participation in the behaviors one week later at time 2. Regressions showed that RAA constructs accounted for good variance in intention and behavior in all behaviors. Intention towards diet and blood glucose monitoring was significantly predicted by instrumental attitude, injunctive norm, and capacity. Intention towards physical activity was significantly predicted by instrumental attitude, experiential attitude, injunctive norm, capacity, and autonomy. All behaviors were significantly predicted by intention, action planning, and coping planning. Additionally, capacity significantly predicted physical activity and autonomy significantly predicted diet and blood glucose monitoring. Successfully intervening in the influential psychological constructs identified in the study could ensure optimal blood glucose regulation in those managing diabetes.