This study examined the effectiveness of Kg-Free: an acceptance-, mindfulness- and compassion-based group intervention for women with overweight and obesity at post-treatment and 3-month follow-up and explored the psychological processes that underlie changes in quality of life, weight self-stigma, body mass index and emotional eating at post-treatment. Overall, 53 women completed Kg-Free. At post-treatment and 3-month follow-up, participants reported increased quality of life, mindfulness and self-compassion abilities and decreased weight self-stigma, emotional eating, shame, weight-related experiential avoidance, self-criticism and body mass index. Shame and self-criticism reductions were important mediators of changes in health-related outcomes, whereas weight-related experiential avoidance, mindfulness and self-compassion mediated changes in weight and eating-related outcomes.