Objectives: Predictors of success among emerging adults (EAs; ages 18-25) within behavioral weight loss (BWL) trials are largely unknown. We examined whether early program engagement predicted overall engagement and weight loss in EAs. Methods: Data were pooled from 2 randomized controlled pilot trials in EAs. Participants (N = 99, 80% female, BMI = 33.7±5.1 kg/m²) received a 3-month BWL intervention. Weight was objectively assessed at 0 and 3 months; engagement was tracked weekly; retention was assessed at 3 months. Results: Greater engagement during the initial 4 weeks of treatment predicted greater weight loss (p = .001). Compared to those who did not engage in all 4 initial weeks, participants meeting this threshold experienced greater overall engagement (9.6 vs 4.2 weeks, p < .001), weight losses (intent-to-treat = -3.8% vs -1.3%, p = .004), and retention (78% vs 53%, p = .012). Conclusions: Early engagement in BWL is associated with better outcomes among EAs. Monitoring engagement in real-time during the initial 4 weeks of treatment may be necessary to intervene effectively. Early engagement did not vary by sex or race; future work should identify characteristics associated with poor early engagement.