Little is known about the assessment and treatment of self-stigma in substance abusing populations. This article describes the development of an acceptance based treatment (Acceptance and Commitment Therapy–ACT) for self-stigma in individuals in treatment for substance use disorder. We report initial outcomes from a study with 88 participants in a residential treatment program. The treatment involves 6 h of a group workshop focused on mindfulness, acceptance, and values work in relation to self-stigma. Preliminary outcomes showed medium to large effects across a number of variables at post-treatment. Results were as expected with one potential process of change, experiential avoidance, but results with other potential mediators were mixed.