This study evaluated a 4-week web-based Acceptance and Commitment Therapy (ACT) mental health promotion intervention for university students.Participants were randomized to intervention (n = 596) or waitlist control ( n = 566). Assessment of primary outcomes (depression, anxiety, stress, well-being, self-compassion, life satisfaction, and academic performance) and ACT processes (acceptance, cognitive fusion, education values, valued living, and present moment awareness) occurred at pre- and post-intervention and 12-week follow-up for intervention participants, and the same pre-post interval for waitlist control participants.Analyses showed significant improvements from pre- to post-intervention compared with waitlist control on all primary outcomes and ACT processes. All intervention gains were maintained at follow-up. Improvements on all primary outcomes were mediated by three or more ACT processes in both samples. Intervention effects were consistent across both sample groupings.Findings provide support for a web-based ACT mental health promotion intervention for university students.