AIMS: To evaluate the immediate and six-month effectiveness of a group-based self-management support program for people diagnosed with type 2 diabetes (1-3 years post diagnosis) on diabetes self-care, distress and cognitions. METHODS: People with type 2 diabetes were randomized into the intervention (four group-based interactive sessions) or the control group (a single educational lecture) with their partners. Outcomes were measured at baseline, immediately after the third course session and six months later using validated questionnaires on diabetes self-care, distress, illness perceptions, diabetes-related attitudes, empowerment and partner support. Multilevel analyses were conducted according to the intention-to-treat principle using the data from 82 intervention and 86 control group participants, to test for differences in changes over time between the two groups. RESULTS: The intervention group showed a significantly higher increase in physical activity and fruit and vegetable intake immediately after the program, whereas the low baseline levels of diabetes distress remained unaffected. Furthermore, the intervention group believed their illness to be more likely to be caused by chance/bad luck, but also felt more empowered to handle their condition and its treatment immediately after the program compared with the control group. Six months later, only the differences in empowerment had persisted. CONCLUSIONS: Group-based self-management support results in favorable short-term behavioral changes and more persistent alterations in (perceived) empowerment in people living in the first years of type 2 diabetes. In order to achieve more sustainable behavioral changes, more prolonged support is necessary. This could be achieved by integrating attention to patients' illness perceptions and continuous self-management support in regular diabetes care. TRIAL REGISTRATION: Netherlands Trial Registry NL3158.