This prospective study assessed the effects of diaphragmatic breathing and systematic relaxation on depression, anxiety, and stress levels, as well as glycemic control, in patients with type 2 diabetes mellitus (T2DM). One hundred patients with T2DM were randomly assigned to two equal groups: Group A patients received conventional treatment for T2DM, and Group B patients received conventional treatment for T2DM plus training in diaphragmatic breathing and systematic relaxation and home practice of these stress-management techniques for 6 months. Stress, depression, and anxiety levels, blood sugar, and glycated hemoglobin (HbA1c) were recorded at baseline and after 6 months of treatment in all patients. Baseline characteristics were compared using the chi-square test and student’s t test. Changes in mental well-being and glycemic status were assessed for their significance in each group using student’s t test and compared between two groups using one-way analysis of covariance (ANCOVA). Baseline levels of the respective change outcome and duration of diabetes were used as covariates in the ANCOVA. A significant decrease was seen in depression, anxiety, and stress scores in Group B, but in Group A only the stress score decreased after 6 months. A significant decline occurred in blood sugar (fasting, 2-hour postprandial, and random) and HbA1c in both groups after 6 months. There was a larger decrease in depression and anxiety scores and HbA1c in Group B than in Group A. The decrease in HbA1c was significantly correlated with the decrease in anxiety and stress scores in both groups and with the depression score in Group A. Thus, the addition of diaphragmatic breathing and systematic relaxation to conventional T2DM treatment appears to have led to improvement in mental well-being and glycemic control in patients with T2DM.