Introduction: the prevalence of diabetes mellitus in Mexican population is 12 %. Hemoglobin A1C (HbA1c) is the most important indicator of control of this disease, so it is required to develop psychological interventions to help people to control HbA1c. Objective: develop and evaluate a positive and cognitive behavioral intervention to control HbA1c in people with type 2 diabetes mellitus (T2DM). Method: eleven patients with T2DM were randomly assigned to an experimental group (EG; n = 6) or control group (CG; n = 5). The intervention consisted of 12 weekly sessions. Self-efficacy, hope, resilience, and optimism were measured with the Diabetes Treatment Self-Efficacy Instrument, Adult Hope Scale, Mexican Resilience Scale, and Life Orientation Test, respectively, along with an assessment of HbA1c, after intervention, and at a three-month follow-up. Results: clinically significant improvements on HbA1c after intervention (g = 0.426) and follow-up (g = 0.749), improvements in self-efficacy after intervention (g = 1.168) and follow-up (g = 0.873) were identified in EG. Discussion: clinically favorable, but not statistically significant, effects of the intervention were discussed; in addition, the role of study variables in HbA1c control is discussed. Conclusions: positive and cognitive behavioral intervention is a promising alternative to improve HbA1c and self-efficacy in people with T2DM. Thus, it is advisable to conduct further evaluation with randomized controlled trials.