This study evaluated the effectiveness of 6 to 10 sessions of Acceptance and Commitment Therapy (ACT) for self-stigma around sexual orientation linked to same-sex attraction (what has generally been referred to as internalized homophobia; IH) in a concurrent multiple-baseline across-participants design. Three men and 2 women showed sizeable reductions from baseline to posttreatment and to 4- and 12-week follow-ups in daily reports of the degree to which thoughts about sexual orientation interfered in their lives; distress associated with these thoughts also decreased. Positive changes were observed in self-report measures of IH, depression, anxiety, stress, quality of life, and perceived social support. Consistent with the theory underlying ACT, reductions in daily ratings of the believability of thoughts about same-sex attraction (a process variable) were greater than those observed for frequency of such thoughts. Improvements were also observed in questionnaires measuring ACT processes. Mixed regression analyses confirmed outcome and process effects that were apparent through visual inspection. Implications and the distinctiveness of ACT as an approach are discussed. (Contains 3 tables and 2 figures.)