Introduction The optimal intravenous fluid regimen for patients undergoing major abdominal surgery is unclear. However, results from many small studies suggest a restrictive regimen may lead to better outcomes. A large, definitive clinical trial evaluating perioperative fluid replacement in major abdominal surgery, therefore, is required. Methods/analysis We designed a pragmatic, multicentre, randomised, controlled trial (the RELIEF trial). A total of 3000 patients were enrolled in this study and randomly allocated to a restrictive or liberal fluid regimen in a 1:1 ratio, stratified by centre and planned critical care admission. The expected fluid volumes in the first 24â €..hour from the start of surgery in restrictive and liberal groups were ≤3.0â €..L and ≥5.4â €..L, respectively. Patient enrolment is complete, and follow-up for the primary end point is ongoing. The primary outcome is disability-free survival at 1â €..year after surgery, with disability defined as a persistent (at least 6â €..months) reduction in functional status using the 12-item version of the World Health Organisation Disability Assessment Schedule. Ethics/dissemination The RELIEF trial has been approved by the responsible ethics committees of all participating sites. Participant recruitment began in March 2013 and was completed in August 2016, and 1-year follow-up will conclude in August 2017. Publication of the results of the RELIEF trial is anticipated in early 2018. Trial registration number ClinicalTrials.gov identifier NCT01424150.