Lesbian, gay, bisexual, and transgender (LGBT) people seek psychotherapy at high rates, and the importance of providing culturally appropriate and LGBT-affirmative psychotherapy has been widely acknowledged. Despite this, remarkably little research has investigated the effects of therapist training in LGBT-affirmative psychotherapy. Here we examined the effectiveness of a training protocol for LGBT-affirmative psychotherapy with 96 mental health professionals, ranging in therapeutic experience from <1 year to 37 years (M = 6.54 years). Across all outcomes, therapists reported increased knowledge and skills related to working with LGBT clients following the training. Therapists also displayed reductions in homo-negativity and trans-negativity. Therapists’ characteristics did not influence the extent to which they benefited from training. Specifically, years of clinical experience, therapist religiosity, and therapist psychological flexibility were unrelated to changes in attitudes, knowledge, and skills. The results of this study clearly suggest that providing training in LGBT-affirmative psychotherapy can enhance therapists’ attitudes, knowledge, and skills. Of particular importance is that the benefits associated with such training appear to hold regardless of therapists’ characteristics.