The levels of psychological distress and burnout among healthcare staff are high, with negative implications for patient care. A growing body of evidence indicates that workplace programmes based on Acceptance and Commitment Therapy (ACT) are effective for improving employees’ general psychological health. However, there is a paucity of research examining the specific psychological and/or behavioural processes through which workplace ACT programmes transmit their beneficial effects. The aim of this randomised controlled trial was to investigate the outcomes and putative processes of change in a 4-session ACT training programme designed to reduce psychological distress among healthcare staff (n = 98). Ninety-eight employees of a healthcare organisation were randomly allocated to the ACT intervention or to a waiting list control group. Study measures were administered on four occasions (baseline, mid-intervention, post-intervention, and follow-up) over a three-month evaluation period. Results showed that the ACT intervention led to a significant decrease in symptoms of psychological distress and a less pronounced reduction in burnout. These effects were mediated primarily via an improvement in mindfulness skills and values-based behaviour and moderated by participants’ initial levels of distress. At four-week post-intervention, 48% of participants who received the ACT intervention showed reliable improvements in psychological distress, with just under half of the aforementioned improvements (46.15%) meeting criteria for clinically significant change. The results advance ACT as an effective stress management intervention for healthcare staff. The findings should be confirmed in a large scale randomised controlled trial with longer follow-up and cost-effectiveness analyses.