Background: Third-wave therapies appear to produce positive outcomes for people without intellectual disabilities. This systematic review aimed to establish which third-wave therapies have been adapted for adults with intellectual disabilities and whether they produced positive outcomes. Method: Four databases were searched systematically (PsycINFO, Web of Science, MEDLINE and PubMed), yielding 1,395 results. Twenty studies (N = 109) met the present review's inclusion/exclusion criteria. Results: Included studies used mindfulness-based approaches, dialectical behaviour therapy, compassion focused therapy and acceptance and commitment therapy. Due to considerable heterogeneity in the designs and outcome measures used, a meta-analysis was not possible. Conclusions: Evidence indicated that third-wave therapies improved mental health symptoms for some and improved challenging/offending behaviour, smoking and mindfulness/acceptance skills for most. These findings must be interpreted with caution due to the low methodological quality of included studies. Future research should build on the current evidence base, using scientifically rigorous designs and standardized measures.