Almost 70% of patients with mental disorders report sleep difficulties and 30% fulfill the criteria for insomnia disorder. Cognitive behavioral therapy for insomnia (CBT-I) is the first-line treatment for insomnia according to current treatment guidelines. Despite this circumstance, insomnia is frequently treated only pharmacologically especially in patients with mental disorders. The aim of the present meta-analysis was to quantify the effects of CBT-I in patients with mental disorders and comorbid insomnia on two outcome parameters: the severity of insomnia and mental health. The databases PubMed, CINHAL (Ebsco) und PsycINFO (Ovid) were searched for randomized controlled trials on adult patients with comorbid insomnia and any mental disorder comparing CBT-I to placebo, waitlist or treatment as usual using self-rating questionnaires as outcomes for either insomnia or mental health or both. The search resulted in 1994 records after duplicate removal of which 22 fulfilled the inclusion criteria and were included for the meta-analysis. The comorbidities were depression (eight studies, 491 patients), post-traumatic stress disorder (PTSD, four studies, 216 patients), alcohol dependency (three studies, 79 patients), bipolar disorder (one study, 58 patients), psychosis (one study, 50 patients) and mixed comorbidities within one study (five studies, 189 patients). The effect sizes for the reduction of insomnia severity post treatment were 0.5 (confidence interval, CI, 0.3-0.8) for patients with depression, 1.5 (CI 1.0-1.9) for patients with PTSD, 1.4 (CI 0.9-1.9) for patients with alcohol dependency, 1.2 (CI 0.8-1.7) for patients with psychosis/bipolar disorder, and 0.8 (CI 0.1-1.6) for patients with mixed comorbidities. Effect sizes for the reduction of insomnia severity were moderate to large at follow-up. Regarding the effects on comorbid symptom severity, effect sizes directly after treatment were 0.5 (CI 0.1-0.8) for depression, 1.3 (CI 0.6-1.9) for PTSD, 0.9 (CI 0.3-1.4) for alcohol dependency in only one study, 0.3 (CI -0.1 - 0.7, insignificant) for psychosis/bipolar, and 0.8 (CI 0.1-1.5) for mixed comorbidities. There were no significant effects on comorbid symptoms at follow-up. Together, these significant, stable medium to large effects indicate that CBT-I is an effective treatment for patients with insomnia and a comorbid mental disorder, especially depression, PTSD and alcohol dependency. CBT-I is also an effective add-on treatment with the aim of improving mental health in patients with depression, PTSD, and symptom severity in outpatients with mixed diagnoses. Thus, in patients with mental disorders and comorbid insomnia, given the many side effects of medication, CBT-I should be considered as a first-line treatment.