Trichotillomania is characterized by recurrent pulling out of one's hair, leading to significant hair loss and accompanied by clinically significant distress and/or functional impairment. The current study used data from a randomized controlled trial comparing the effectiveness of acceptance-enhanced behavior therapy (AEBT) to psychoeducation plus supportive therapy (PST; active control) for trichotillomania in an adult sample. The objectives were to examine the moderating and mediating influence of trichotillomania-specific psychological flexibility in treatment for trichotillomania. Participants with lower baseline flexibility performed better in AEBT than PST in terms of greater symptom reduction and quality of life. Lower baseline flexibility also predicted higher likelihood of disorder recovery in AEBT relative to PST. In addition, relative to PST, symptom reduction in AEBT was mediated by psychological flexibility, controlling for anxiety and depression. These findings suggest that psychological flexibility is a relevant process of change in the treatment of trichotillomania. Clinical implications and directions for future research are discussed.