Background: Based on the proven role of sleep quality in promoting health, the treatment of insomnia should be premised on identification of characteristics and severity of insomnia symptoms. This study aimed to investigate the effectiveness of Cognitive-Behavioral Therapy for Insomnia (CBT-I) in improving sleep quality and reducing insomnia severity among the afflicted patients. Methods: This was a semi-experimental study with a pre-test, post-test, and follow-up design and control group. The statistical population included all patients visiting the Sleep Disorder Clinic of Rasoul Akram Hospital and two private psychiatric clinics, selected using convenience sampling in Tehran in 2020. Twenty out of 60 patients with insomnia disorder, diagnosed based on the Diagnostic and Statistical Manual for Insomnia, were randomly selected using a lottery method. The participants were divided into the experimental and control groups. In the cognitive-behavioral group, each participant individually received a 6-week treatment intervention. All participants in both groups were evaluated at the beginning of the treatment, at the end of the treatment, and one month after the treatment. The Pittsburgh Sleep Quality Index (PSQI) and Insomnia Severity Index (ISI) were used for data collection. Results: The results showed a significant between-group difference in the scores of sleep quality and insomnia severity at post-test stage (p