A novel eight-week program based on Acceptance and Commitment Therapy (ACT) principles was created to alleviate burnout-related ill-being and to enhance well-being. This randomized controlled trial investigated the effectiveness of the program and explored whether changes in psychological flexibility mediated the results of the intervention. The program consisted of structured weekly face-to-face group meetings and daily practices provided via a website. Employees from varying professional backgrounds with burnout (mean age = 47 years, 79% female), who all received usual treatment, 1 were randomized into control (TAU, n = 80, receiving no other support) and ACT + TAU intervention (n = 88, receiving additional ACT support) groups. The ACT + TAU group outperformed the TAU group in all 14 scales used, indicating that burnout-related ill-being at work (between-group Cohen's d = 0.36–0.76) and psychological symptoms (d = 0.27–0.61) decreased and general well-being (d = 0.14–0.38) and psychological flexibility skills (d = 0.29–0.64) increased during the intervention. These gains were maintained during the one-year follow-up period. The changes in the psychological flexibility -factor mediated almost completely the changes in the outcome factors of burnout, well-being, and psychological symptoms. The study suggests that psychological flexibility skills can be crucial elements in job-related burnout interventions and that combined group and web-based interventions may offer an efficient treatment method.