Aims: This paper aims to explore the effect of the social network based intervention Powerful Together with Diabetes on diabetes self-management among socioeconomically deprived patients. This 10-month group intervention targeting patients and significant others aimed to improve self-management by stimulating social support and diminishing social influences that hinder self-management. Methods: This intervention was evaluated in a quasi-experimental study using a mixed methods approach. Of 131 socioeconomically deprived patients with suboptimal glycaemic control, 69 were assigned to the intervention group and 62 to the control group (standard diabetes education). 27 qualitative in-depth interviews with the participants and 24 with their group leaders were held to study the subjective impact of the intervention. Further, self-management behaviours (medication adherence, diet and physical activity) were assessed at baseline, 10 and 16 months. Data were analysed using framework analyses and a linear mixture model. Results: Qualitative data showed that the intervention group had a better understanding of the way self-management influences diabetes. The intervention group showed more complex self-management behaviours, such as planning ahead, seeking adequate food and physical activity alternatives, and consistently taking their diabetes into consideration when making choices. In participants with complete follow-up data, we found a significant increase in physical activity in the intervention group (3.78 vs. 4.83 days) and no changes in medication adherence and diet. Conclusions: This study indicates that an intensive support group and simultaneously involving significant others might improve diabetes self-management behaviours among socioeconomically deprived patients. More studies are needed to justify further implementation of the intervention. This study is registered in the Dutch Trial Register NTR1886. http://www.trialregister.nl/trialreg/admin/rctview.asp?TC=1886