Cognitive-behavioral therapy for insomnia (CBT-I) has been shown efficacious, but the challenge remains to make it available and accessible in order to meet population needs. Delivering CBT-I over the internet (eCBT-I) may be one method to overcome this challenge. The objective of this meta-analysis was to evaluate the efficacy of eCBT-I and the moderating influence of various study characteristics. Two researchers independently searched key electronic databases (1991 to June 2015), selected eligible publications, extracted data, and evaluated methodological quality. Eleven randomized controlled trials examining a total of 1460 participants were included. Results showed that eCBT-I improved insomnia severity, sleep efficiency, subjective sleep quality, wake after sleep onset, sleep onset latency, total sleep time, and number of nocturnal awakenings at post-treatment, with effect sizes (Hedges's g) ranging from 0.21 to 1.09. The effects were comparable to those found for face-to-face CBT-I, and were generally maintained at 4–48 wk follow-up. Moderator analyses showed that longer treatment duration and higher degree of personal clinical support were associated with larger effect sizes, and that larger study dropout in the intervention group was associated with smaller effect sizes. In conclusion, internet-delivered CBT-I appears efficacious and can be considered a viable option in the treatment of insomnia.