Chronic stress increases the risk of health problems and absenteeism, with negative consequences for individuals, organizations and society. The aim of the present study was to examine the effect of a brief stress management intervention based on the principles of Acceptance and Commitment Therapy (ACT) on stress and general mental health for Swedish social workers (n = 106) in a randomized, controlled trial. Participants were stratified according to stress level at baseline in order to examine whether initial stress level moderated the effect of the intervention. Two thirds of the participants had high stress levels at baseline (Perceived Stress Scale; score of >= 25). The results showed that the intervention significantly decreased levels of stress and burnout, and increased general mental health compared to a waiting list control. No statistically significant effects were, however, found for those with low levels of stress at baseline. Among participants with high stress, a substantial proportion (42%) reached criteria for clinically significant change. We concluded that the intervention successfully decreased stress and symptoms of burnout, and increased general mental health. Evidence is, thus, provided supporting ACT as brief, stress management intervention for social workers.