This meta-analysis examines the relationship between depression and treatment nonadherence in patients with type 1 and type 2 diabetes. The study found a significant association between depression and nonadherence to diabetes treatment, with a weighted effect size of $r = 0.21$ (95% CI 0.17–0.25), indicating a moderate effect. The analysis included 47 independent samples from 43 published reports, totaling over 17,000 participants. Moderator analyses revealed that the effect was larger in studies that measured self-care as a continuous variable and in those with stronger methodologies. The strongest effect was observed for missed medical appointments, followed by composite measures of self-care. The findings suggest that depression may represent an important pathway between depression and worse diabetes clinical outcomes, particularly through impaired self-care behaviors. The results also highlight the need for comprehensive interventions that address both depression and self-care to improve diabetes control.This meta-analysis examines the relationship between depression and treatment nonadherence in patients with type 1 and type 2 diabetes. The study found a significant association between depression and nonadherence to diabetes treatment, with a weighted effect size of $r = 0.21$ (95% CI 0.17–0.25), indicating a moderate effect. The analysis included 47 independent samples from 43 published reports, totaling over 17,000 participants. Moderator analyses revealed that the effect was larger in studies that measured self-care as a continuous variable and in those with stronger methodologies. The strongest effect was observed for missed medical appointments, followed by composite measures of self-care. The findings suggest that depression may represent an important pathway between depression and worse diabetes clinical outcomes, particularly through impaired self-care behaviors. The results also highlight the need for comprehensive interventions that address both depression and self-care to improve diabetes control.