This article reviews evaluation studies of programs designed to treat sex offenders with intellectual and developmental disabilities (IDD) published in peer-reviewed journals between 1994 and 2014. The design of this study is mirrored after PRISMA (Preferred Reporting of Items for Systematic Reviews and Meta-Analyses) recommendations for conducting a systematic literature review. The study design, study setting, characteristics of participants, type of treatment, and intervention procedures comprise areas of focus for evaluating the implementation of treatment programs. Therapeutic outcomes include changes in attitudes consistent with sex offending, victim empathy, sexual knowledge, cognitive distortions, and problem sexual behaviors. Eighteen treatment evaluation studies were identified from the United States, the United Kingdom, Australia, and New Zealand. Cognitive-behavioral treatments were the most commonly delivered treatment modality to sex offenders with IDD. Other less common treatments were dialectical behavioral therapy, problem solving therapy, mindfulness, and relapse prevention. No randomized controlled trials were identified. The most common designs were multiple case studies and pre- and post-treatment assessments with no control and repeated measures follow-up. Small sample sizes, no control groups, and wide variation in treatment length and follow-up time complicate the qualitative synthesis of study findings. Short follow-up times introduce the potential for bias in conclusions surrounding treatment efficacy for many of the studies reviewed in this analysis. The overall quality of studies examining treatments for sex offenders with IDD is poor and requires further development before rendering firm conclusions about the effectiveness of interventions for this population.