People with intellectual disabilities (ID) often suffer from psychopathological symptoms and are rarely treated psychotherapeutically. Concurrently, there is a lack of studies summarizing the effectiveness of cognitive-behavioral therapy (CBT) for adults with ID and co-morbid psychopathological symptoms. A literature search identified CBT-trials focusing on the outcome domains anxiety, depression, and anger in adults with ID (IQ 20–69). A meta-analysis with subgroup analyses was conducted. Controlled trials showed significant effects for the outcome domains depression (d = 0.65; eight studies; 95% confidence interval, CI [0.24, 1.07]) and anger (d = 0.65; six studies; 95% CI [0.25, 1.06]), and a small and nonsignificant effect for anxiety (d = 0.25; four studies; 95% CI [0.00, 0.49]). Due to only a moderate number of trials and limited methodological quality, larger and more rigorous trials are needed to support the preliminary evidence for the efficacy of CBT for individuals with ID.