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Objective: The aim of this systematic review and meta-analysis (PROSPERO 2020 CRD42020169323) was to evaluate the efficacy of psychological therapy for people with intellectual disabilities. Method: A comprehensive literature search yielded 22,444 studies which were screened for eligibility. Studies were eligible for inclusion if a psychological therapy was delivered to people with intellectual disabilities compared to a group who did not receive the therapy. Thirty-three controlled trials were eligible for inclusion in the review, with 19 included within a DerSimonian-Laird random effects meta-analysis. Subgroup analysis was completed by clinical presentation, and by comparing randomised trials to non-randomised trials, and group-based to individually delivered psychotherapy. Results: Following the removal of outliers, psychological therapy for a range of mental health problems was associated with a small and significant effect size, g = 0.43, 95% CI [0.20, 0.67], N = 698. There was evidence of heterogeneity and bias due to studies with small sample sizes and a lack of randomisation. Non-randomised studies were associated with a large effect size, g = 0.90, 95% CI [0.47, 1.32], N = 174, while randomised studies were associated with a small effect size, g = 0.36, 95% CI [0.17, 0.55], N = 438, excluding outliers. Individually delivered psychological therapy was associated with a small and non-significant effect size, g = 0.32, 95% CI [−0.01, 0.65], N = 146, while group-based interventions were associated with a small and significant effect size, g = 0.37, 95% CI [0.05, 0.68], N = 361, again, excluding outliers. Psychological therapy for anger was associated with a moderate effect size, g = 0.60, 95% CI [0.26, 0.93], N = 324, while treatment for depression and anxiety was associated with a small and non-significant effect size, g = 0.38, 95% CI [−0.10, 0.85], N = 216, after outliers were removed. Conclusions: Studies are fraught with methodological weaknesses limiting the ability to make firm conclusions about the effectiveness of psychological therapy for people with intellectual disabilities. Improved reporting standards, appropriately powered and well-designed trials, and greater consideration of the nature and degree of adaptations to therapy are needed to minimise bias and increase the certainty of conclusions.