IntroductionMany people quit smoking during pregnancy, but postpartum smoking relapse is common. Maintaining smoking abstinence achieved during pregnancy is key to improving maternal and child health. There are no evidence-based interventions for preventing postpartum smoking relapse. This trial aims to determine whether an intervention to prevent postpartum relapse is effective and cost-effective.Methods and analysisA randomised controlled trial of a complex intervention to prevent postpartum smoking relapse (BabyBreathe), with internal pilot, economic and process evaluations. Participants are adults who are pregnant and who report having quit smoking in the 12 months before, or during pregnancy. Participants are eligible if they read and understand English, and provide informed consent. Following consent and biochemical validation of smoking abstinence, participants are randomised to intervention or usual care/control (no specific relapse prevention support). The BabyBreathe intervention consists of manualised advice from a trained member of the health visiting service, health information leaflets for participants and partners, access to the BabyBreathe website and app. At the time of birth, participants are posted the BabyBreathe box and support is provided by text message for up to 12 months postpartum. Target sample size is 880, recruiting across midwifery services at four hubs in England and Scotland and through remote advertising in England, Scotland, Wales and Northern Ireland. Outcomes are collected at 6 and 12 months. The primary outcome is self-reported sustained smoking abstinence at 12 months, carbon monoxide verified. Secondary outcomes include self-reported abstinence, time to relapse, partner smoking status and quality of life.Ethics and disseminationThe trial was approved by the North West Preston Research Ethics committee (21/NW/0017). Dissemination will include publication in peer-reviewed journals, presentation at academic and public conferences including patient and public involvement and to policymakers and practitioners.Trial registration numberISRCTN70307341