Based on self-determination theory (SDT), this study investigated, whether the three central SDT variables (perceived autonomy support, autonomous motivation and self-care competence), were associated with engagement in physical activity (PA) among patients with type 2 diabetes when the effect of a wide variety of other important life-context factors (perceived health, medication, duration of diabetes, mental health, stress and social support) was controlled for. Patients from five municipalities in Finland with registry-based entitlement to a special reimbursement for medicines used in the treatment of type 2 diabetes (n = 2866, mean age 63 years, 56% men) participated in this mail survey in 2011. Of all measured explanatory factors, autonomous motivation was most strongly associated with engagement in PA. Autonomous motivation mediated the effect of perceived autonomy support on patients’ PA. Thus, perceived autonomy support (from one’s physician) was associated with the patient’s PA through autonomous motivation. This result is in line with SDT. Interventions for improved diabetes care should concentrate on supporting patients’ autonomous motivation for PA. Internalizing the importance of good self-care seems to give sufficient energy to maintain a physically active lifestyle.