Maintenance of weight loss is hard to achieve, and novel interventions are needed to improve long-term outcomes. In this pilot randomized controlled trial, N = 188 participants received an online, 12-week weight loss intervention and N = 102 who lost ≥ 5% were then randomly assigned to a 1-day, 5-h workshop based on Acceptance and Commitment Therapy (ACT), Self-Regulation (SR), or no workshop (Control) with 3 months of limited email follow-up. Assessments were conducted at baseline, 3, 6, 12, 18, and 24 months. The primary outcome was percent weight change; secondary outcomes were weight-related experiential avoidance and health values-consistent behavior. ACT had greater overall weight loss (−7.18%, SE = 1.33) when compared to Control (−1.15%, SE = 1.50; p = .03). Post hoc analyses showed that ACT had significantly greater weight losses than Control (6.11%, β = −2.03, p = .048) among those with lower initial weight loss (5–7%), and significantly greater weight loss than SR (6.19%, β = −1.77, p = .05) among those with the highest initial weight losses (10% +). There is potential for continuing to develop ACT in a limited interventionist-contact format with modifications. This pilot study represents an innovative model for behavioral weight loss by reversing the typical treatment intensity model with the aim of providing interventionist support during a critical period after initial weight loss.