Introduction: Guidelines recommend cognitive behavioral therapy for insomnia (CBT-I) as the first line of treatment for insomnia in general practice, but CBT-I is rarely available. Nurse-guided Internet-delivered CBT-I might be a solution to improve access to care. Objective: We aimed to determine the effectiveness of nurse-guided Internet-delivered CBT-I (I-CBT-I) on insomnia severity experienced by patients in general practice. Methods: Nurse-guided I-CBT-I ("i-Sleep") was compared to care-as-usual (and I-CBT-I after 6 months) in 15 participating general practices among 134 patients (≥18 years old) with clinical insomnia symptoms. Assessments took place at 8, 26 and 52 weeks. Primary outcome was self-reported insomnia severity (Insomnia Severity Index) at 8 weeks. Secondary outcomes were sleep diary indices, depression and anxiety symptoms (Hospital Anxiety and Depression Scale), fatigue, daytime consequences of insomnia, sleep medication and adverse events. Results: Two thirds of the 69 intervention patients (n = 47; 68%) completed the whole intervention. At the posttest examination, there were large significant effects for insomnia severity (Cohen's d =1.66), several sleep diary variables (wake after sleep onset, number of awakenings, terminal wakefulness, sleep efficiency, sleep quality) and depression. At 26 weeks there were still significant effects on insomnia severity (d = 1.02) and on total sleep time and sleep efficiency. No significant effects were observed for anxiety, fatigue, daily functioning or sleep medication. No adverse events were reported. Conclusions: Nurse-guided I-CBT-I effectively reduces insomnia severity among general practice patients. I-CBT-I enables general practitioners to offer effective insomnia care in accordance with the clinical guidelines.