Background: Although Acceptance and Commitment Therapy (ACT) may be effective for individuals with psychosis and a history of childhood trauma, little is known about predictors of treatment response among such patients. Aims: The current study examined: (1) whether severity of trauma predicted treatment response, and (2) profiles of patients with regard to their responses to treatment. Method: Fifty participants with psychosis and childhood trauma history were recruited and randomized to take part in either eight sessions of group-based ACT, or to be on a waiting list for the ACT group (i.e. treatment as usual group). The entire sample was used for the first part of the analyses (aim 1), whereas subsequent subsample analyses used only the treatment group (n = 30 for aim 2). Results: It was found that trauma severity did not moderate the effectiveness of ACT on symptom severity, participants’ ability to regulate their emotional reactions, or treatment compliance with regard to help-seeking. In addition, among those receiving ACT, the results revealed three distinct and clinically relevant change profiles. Avoidant attachment style and number of sessions attended were predictive of belonging to the different clusters or profiles. Patients in the profile representing the least amount of clinical change attended an average of two sessions less than those in the other change profiles. Conclusion: ACT offered in a group format appears to be a promising treatment for those with psychosis and history of trauma regardless of trauma severity. Given the brevity of the intervention, patients should be encouraged to attend each session to obtain maximum benefit.