SummaryMany adults suffer from insomnia. Cognitive‐behavioural therapy for insomnia is the recommended treatment option, but access to it is not readily available. Digital interventions have the potential to close the treatment gap by offering scalable and cost‐efficient options. The present randomised controlled trial aimed at investigating the effectiveness and safety of somnovia, an interactive internet‐based intervention for patients with insomnia. A total of 290 participants with chronic insomnia were randomised to intervention (n = 149) or the control (n = 141) condition. Participants of the intervention group received access to somnovia for 6 months in addition to treatment as usual, whereas participants in the control group only had access to treatment as usual for the time of the study. Online questionnaires were filled in before randomisation and after 3 and 6 months. The primary endpoint was the Insomnia Severity Index, with the Patient Health Questionnaire‐9, the Generalised Anxiety Disorder Assessment‐7, and the Work and Social Adjustment Scale as secondary endpoints. After 3 months, the intervention group showed lower insomnia (Cohen's d = 0.71, CI = [0.44, 0.98]), depressive (Cohen's d = 0.66, CI = [0.41, 0.90]), and anxiety (Cohen's d = 0.56, CI = [0.32, 0.81]) symptoms, as well as improved overall functioning (Cohen's d = 0.50, CI = [0.24, 0.76]) compared with participants in the control group. The effects stayed stable after 6 months. The results indicate that next to a therapeutic effect on insomnia symptoms, somnovia might potentially help to prevent the onset of other psychiatric disorders such as depression and anxiety.