Background: Data from sex offender treatment programmes (SOTP) on the re-offending of convicted sex offenders from the prison service in England and Wales was recently reviewed and found to be associated with no change in sexual reoffending. While this result is at variance with a number of other reviews it does give rise to concern as most programmes for individuals with intellectual disabilities adopt similar treatment methods. It is therefore important to consider the treatment outcomes for this client group by conducting a meta-analysis to examine treatment effects. Method: A systematic search of PsychINFO, EMBASE, MEDLINE and CINAHL was conducted in February 2019 using specific inclusion and exclusion criteria. Eighteen papers were identified, and their quality was assessed using a risk of bias framework. Scores on cognitive distortions, sexual knowledge and victim empathy measures as well as reports of further sexually abusive behaviour were pooled together to run a meta-analysis which yielded effect sizes for each outcome. Results: A large treatment effect was found for reducing cognitive distortions and increasing victim empathy, with a moderate effect found for improvements in sexual knowledge. This meta-analysis also found that 11.5% of individuals with intellectual disabilities who have completed these programmes will go on to display further sexually abusive behaviours. Conclusions: The findings suggest that cognitive distortions are significantly reduced following treatment, there are also significant improvements in sexual knowledge and victim empathy. A rate of 11.5% for further reported sexually abusive behaviour during follow up periods was found, which is higher than the rate found for mainstream SOTP completers. Problems regarding the recording of sexually abusive behaviour are considered.