© 2016 American Psychological Association. Objective: For people living with HIV, treatment adherence self-efficacy is an important predictor of treatment adherence and, therefore, of clinical outcomes. Using experience sampling method (ESM), this study aimed to examine: (1) the within-person association between moment-to-moment changes in social support and HIV treatment self-efficacy; (2) the moderators of this within-person association; (3) the concordance between questionnaire and ESM measurement of treatment self-efficacy; and (4) the utility of each approach (ESM and questionnaire) in predicting adherence to medication, adherence to clinic visits, CD4 counts, and viral load. Method: Men living with HIV (N = 109) responded to the same set of ESM questions 3 times a day for 7 days via a smart phone given to them for the study. They also completed cross-sectional questionnaires and their clinic data was extracted from medical records to examine predictors and consequences of state and trait treatment self-efficacy. Results: In within-person hierarchical linear modeling (HLM) analyses, receipt of recent social support predicted higher current ESM treatment self-efficacy. This association was stronger for individuals reporting higher avoidance coping with HIV. The correlation between ESM and questionnaire measures of treatment self-efficacy was r = .37. ESM measure of average treatment self-efficacy predicted medication adherence, visit adherence, CD4 counts, and viral load, while questionnaire-based self-efficacy did not predict these outcomes. Conclusion: Interventions aimed at improving treatment adherence may target social support processes, which may improve treatment self-efficacy and adherence.