This systematic review and meta-analysis assessed the efficacy of cognitive-behavioural therapy for insomnia (CBT-I) among cancer survivors and explored its effect on comorbid symptoms. Studies were included if they assessed the efficacy of CBT-I in adults diagnosed with cancer published prior to August 2020. The primary outcome was insomnia severity. The protocol was pre-registered on PROSPERO (CRD42020169986). Twenty-two studies met eligibility criteria. CBT-I significantly improved insomnia severity (g = 0.78) with durable benefits at 3- and 6-month follow-up. CBT-I produced significant small to large effects for diary-measured sleep efficiency, wake after sleep onset, total sleep time, sleep onset latency, sleep quality, anxiety, depression, fatigue, and overall quality of life. Subgroup analyses revealed no significant difference between in-person and self-help CBT-I. Overall, CBT-I is a robustly efficacious and durable treatment for insomnia among cancer survivors and can produce concomitant benefits on other symptoms. Implementation efforts are needed to ensure that people with cancer have access to CBT-I as the recommended first-line treatment for insomnia.