Purpose: Insomnia has adverse effects on people with epilepsy. We aimed to test a novel cognitive behavioral therapy for insomnia (CBT-I) app-based intervention on insomnia symptoms and social psychological factors in people with epilepsy and to examine the possible mechanisms among the factors. Methods: Participants were recruited from neurology clinics in Iran and comprised individuals diagnosed with epilepsy and having moderate to severe insomnia. A two-arm randomized controlled trial design was used, consisting of a treatment group (CBT-I; n = 160) and control group (patient education; n = 160). Primary outcomes were self-reported sleep quality, insomnia severity, and sleep hygiene behavior and objective sleep characteristics measured by actigraphy. Secondary outcomes were attitude, perceived behavioral control, intention, action planning, coping planning, behavioral automaticity, self-monitoring, anxiety, depression, and quality of life (QoL). All outcomes were measured at baseline, and at one, three, and six months postintervention, except objective sleep, which was assessed at baseline, and one and six months postintervention. Data were analyzed using linear mixed models. Results: Current findings showed that sleep quality, insomnia severity, sleep hygiene behavior, and sleep onset latency were significantly improved in the CBT-I group compared with the patient education group at all measurement points. Also, the CBT-I group had significantly improved anxiety, depression, and QoL compared with the patient education group. Mediation analyses showed that attitude, intention, coping planning, self-monitoring, and behavioral automaticity significantly mediated the effect of the intervention on sleep outcomes. Conclusion: Results support the use of the CBT-I app to improve sleep outcomes among people with epilepsy. (C) 2020 Elsevier Inc. All rights reserved.