IntroductionThe aim of the STOPPIT-3 study is to determine the clinical and cost effectiveness of antenatal corticosteroids (ACS) prior to planned birth of twins in a multicentre placebo-controlled trial with internal pilot.Methods and analysisThis study will comprise a multicentre, double-blinded, randomised, placebo-controlled trial in at least 50 UK obstetric units. The target population is 1552 women with a twin pregnancy and a planned birth between 35 and 38+6 weeks’ gestation recruited from antenatal clinics. Women will be randomised to Dexamethasone Phosphate (24 mg) or saline administered via two intramuscular injections 24 hours apart, 24–120 hours prior to scheduled birth.OutcomesThe primary outcome is need for respiratory support within 72 hours of birth. Secondary and safety outcomes will be included. Cognitive and language development at age 2 years will be assessed in a subset of participants using the Parent report of Children’s Abilities-Revised questionnaire. We will also determine the cost effectiveness of the treatment with ACS compared with placebo.Ethics and disseminationSTOPPIT-3 has been funded and approved by the National Institute of Healthcare Research. It has been approved by the West Midlands Research Ethics Committee (22/WM/0018). The results will be disseminated via publication in peer-reviewed journals and conference presentation and will also be communicated to the public via links with charity partners and social media.Trial sponsorThe University of Edinburgh and Lothian Health Board ACCORD, The Queen’s Medical Research Institute, 47 Little France Crescent, Edinburgh, EH16 4TJ.Trial registration numberISRCTN59959611.