Insomnia and depression are highly comorbid conditions that show a complex, bidirectional relationship. This study examined whether cognitive-behavioral therapy for insomnia (CBT-I) delivered by a therapist compared with self-help CBT-I (written materials only) reduces insomnia and depression severity in individuals with comorbid insomnia and depression. A total of 41 participants (18-64 years; 25 females) with comorbid depression and insomnia, treated with antidepressants for at least 6 weeks, were randomized to receive 4 sessions of either CBT-I or self-help CBT-I over 8 weeks. Insomnia (Insomnia Severity Index ISI) and depression (Beck Depression Inventory-II BDI-II) were assessed at baseline, following each session, and at 3-month follow-up. Secondary outcomes were sleep quality and duration (actigraphy and diaries), anxiety, fatigue, and daytime sleepiness. Compared with self-help CBT-I, BDI-II scores in the CBT-I group dropped by 11.93 (95% confidence interval CI 6.60, 17.27, p < .001) more points, and ISI scores dropped by 6.59 (95% CI 3.04, 10.15, p = .001) more points across treatment. At 3-month follow-up, 61.1% of CBT-I participants were in clinical remission from their insomnia and depression, compared with 5.6% of the self-help group. Conclusions: CBT-I administered by a therapist produced significant reductions in both insomnia and depression severity posttreatment and at follow-up, compared with a control condition in which participants received only written CBT-I material. Targeting insomnia through CBT-I is efficacious for treating comorbid insomnia and depression, and should be considered an important adjunct therapy for patients with depression whose symptoms have not remitted through antidepressant treatment.