We meta-analyzed the relationship between depression and HIV medication nonadherence to calculate the overall effect size and examine potential moderators. Overall, across 95 independent samples, depression was significantly (P < 0.0001) associated with nonadherence (r = 0.19; 95% confidence interval = 0.14 to 0.25). Studies evaluating medication adherence via interview found significantly larger effects than those using self-administered questionnaires. Studies measuring adherence along a continuum found significantly stronger effects than studies comparing dichotomies. Effect size was not significantly related to other aspects of adherence or depression measurement, assessment interval (ie, cross-sectional vs. longitudinal), sex, IV drug use, sexual orientation, or study location. The relationship between depression and HIV treatment nonadherence is consistent across samples and over time, is not limited to those with clinical depression, and is not inflated by self-report bias. Our results suggest that interventions aimed at reducing depressive symptom severity, even at subclinical levels, should be a behavioral research priority.