Objective: Insomnia is a major health problem, and the need for effective and accessible treatment is urgent. The aim of the current study was to evaluate the short-term efficacy of an unguided Internet-based cognitive-behavioral treatment program for insomnia (CBTi), called SHUTi (Sleep Healthy Using the Internet). Methods: This study used a parallel arm randomized controlled trial in Norway. Participants were randomly allocated to the SHUTi condition or a Web-based patient education condition. Both groups were assessed before and after the nine-week intervention period (online sleep diaries and questionnaires). The SHUTi participants were reassessed in a six-month nonrandomized follow-up. Primary outcome measures were the Insomnia Severity Index (ISI) and the Bergen Insomnia Scale (BIS). Results: A total of 181 participants were included in the study; SHUTi condition (n = 95), patient education condition (n = 86). Intention-to-treat mixed-model repeated-measures analysis revealed that the SHUTi group had better short-term outcomes compared with the patient education group on most sleep measures. The SHUTi group showed a significant decrease on the primary outcomes, the ISI (d_between = –1.77, 95% CI = –2.23, –1.31) and the BIS (d_between = –1.00, 95% CI = –1.32, –.68). Improvements were maintained among the completing SHUTi participants at the six-month nonrandomized follow-up. However, dropout attrition was high. Conclusion: Unguided Internet-based CBTi produced significant short-term improvements in sleep in patients with chronic insomnia. This highlights the benefits of making Internet-delivered CBTi programs available as a standard first-line treatment option in public health services. Nevertheless, the rate of dropout attrition (participants not completing post-assessment) in this trial limits the generalizability of the findings.