Background. Diabetes mellitus (DM) is currently not curable but controllable and must be managed. Family support has been stated as a vital component in successful control of DM and the strongest determinant of adherence to diet therapy in DM patients.Objectives. The study was conducted to examine social support and its relationship with practicing self-care behavior in patients with DM in Mashhad. Material and methods. In this analytical cross-sectional study, 400 patients with DM were selected over two months from DM clinics in Mashhad using the census sampling method. Data was collected using demographic information, social support and self-care questionnaires. Data analysis was done in SPSS 21 using the statistical Spearman's rank correlation coefficient, t-Test and analysis of variance. Results. The results indicated a significant relationship between the social support and self-care behavior of DM patients (p = 0.031). This means that social support is an effective factor in practicing self-care behaviors. Moreover, the results indicated that most of the samples had moderate social support and self-care behavior. There was a significant relationship between self-care and health literacy in the study (p = 0.026): individuals with higher health literacy had better self-care. There was a significant relationship between education and monthly income with self-care behaviors and support of the individuals (p = 0.001). Conclusions. Individuals with better social support have adhered to DM self-care behaviors, and adhering to self-care behaviors has led to blood sugar control and prevention of adverse effects. Furthermore, patient support and participation in self-care enable them to take the next step for better management of the disease and prevention of future adverse effects.