This non-randomized, assessor blind study evaluated the effects of cognitive behavioral therapy for insomnia (CBT-I) delivered in a group format on insomnia symptoms as well as psychotic, depressive, and anxiety symptoms in schizophrenia patients (n = 63) recruited from residential or rehabilitative facilities in Seoul, South Korea. Thirty-one patients received four sessions of CBT-I in groups of 2-9 patients in addition to usual care, while the control group (n = 32) received no additional intervention. The Insomnia Severity Index (ISI) and Pittsburgh Sleep Quality Index (PSQI), Psychotic Symptoms Rating Scale (PSYRATS), Anxiety Sensitivity Index (ASI), and Beck Depression Inventory (BDI) were administered at baseline, week 4, and week 8. Both groups showed significant time-group interactions on the ISI and PSQI. Post hoc testing showed that, compared to the control group, the CBT-I group showed significant reductions in ISI and PSQI at both week 4 and week 8. For the PSYRATS, ASI, and BDI scores, the CBT-I and control groups showed significant time-group interactions, but post hoc testing revealed no significant group differences at either week 4 or week 8. Therefore, CBT-I was effective for reducing insomnia symptoms in patients with schizophrenia and the effect lasted for 4 weeks after the intervention.