Objective: To determine whether eating self-efficacy (ESE) and physical activity self-efficacy (PASE) are predictive of dietary intake, physical activity, and weight change within a behavioral weight-loss intervention, and whether dietary intake and physical activity mediate relationships between self-efficacy and weight change.Method: The study sample included 246 participants from a randomized trial with complete data on study variables at 12 months. ESE, PASE, calories consumed, minutes of moderate-to-vigorous physical activity (MVPA), and weight were measured at baseline, 6, and 12 months. Results: ESE at baseline was associated with 12-month percent weight loss (PWL), and was mediated by average calories consumed at 6 and 12 months. Change in ESE from baseline to 6 months was associated with calories consumed at 12 months and 12-month percent weight loss, but the mediated relationship was not significant. Baseline PASE was not associated with average MVPA at 6 and 12 months or 12-month PWL, but change in PASE from baseline to 6 months was associated with 12-month PWL through its effect on MVPA at 12 months. Conclusion: Increases in ESE and PASE during the active phase of the intervention are predictive of dietary intake, physical activity and weight loss at later points, but further research should include explorations of the reciprocal relationship between behavior and self-efficacy to better inform intervention strategies that target self-efficacy and promote behavior change.