The current study examined psychological inflexibility and self-compassion as theoretically relevant mediators and moderators of outcomes following acceptance and commitment therapy (ACT) for clinical perfectionism. Fifty-three participants with clinical perfectionism were randomized to either a 10-session ACT condition or a 14-week waitlist control condition (only 39 completed the post treatment assessment). Outcomes tested include concern over mistakes, doubting of actions, personal standards, quality of life, symptom distress and functional impairment, and valued action. Multilevel modeling analyses showed reduced psychological inflexibility mediated the relationship between condition and higher quality of life and increased self-compassion mediated the relationship between condition and decreased concern over mistakes. No other mediation effects were observed. In addition, baseline psychological inflexibility differentially moderated outcomes depending on outcome tested; for example, lower baseline inflexibility predicted more improvement in quality of life whereas higher baseline inflexibility predicted more improvement in symptom distress and functional impairment. Participants with average baseline self-compassion tended to benefit the most from ACT. These findings clarify how psychological inflexibility and self-compassion influence outcomes following ACT for clinical perfectionism. Theoretical and clinical implications of ACT for clinical perfectionism are discussed.