Background: Prophylactic dressings are increasingly used to prevent pressure injuries in hospitalised patients. However, evidence regarding the effectiveness of these dressings is still emerging. This trial aims to determine the clinical and cost-effectiveness of a prophylactic silicone foam border dressing in preventing sacral pressure injuries in medical-surgical patients. Methods: This is a multicentre, pragmatic, parallel group, randomised controlled trial. A sample size of 1320 was calculated to have >90% power to detect a 5% difference in the primary outcome at an alpha of 0.05. Adult patients admitted to participating medical-surgical wards are screened for eligibility: ≥18 years, admitted to hospital within the previous 36 h, expected length of stay of ≥24 h, and assessed high risk for hospital-acquired pressure injury. Consenting participants are randomly allocated to either prophylactic silicone foam dressing intervention or usual care without any dressing as the control group via a web-based randomisation service independent of the trial. Patients are enrolled across three Australian hospitals. The primary outcome is the cumulative incidence of patients who develop a sacral pressure injury. Secondary outcomes include the time to sacral pressure injury, incidence of severity (stage) of sacral pressure injury, cost-effectiveness of dressings, and process evaluation. Participant outcomes are assessed daily for up to 14 days by blinded independent outcome assessors using de-identified, digitally modified sacral photographs. Those who develop a sacral pressure injury are followed for an additional 14 days to estimate costs of pressure injury treatment. Analysis of clinical outcomes will be based on intention-to-treat, per-protocol, and sensitivity analyses. Discussion: This trial aims to provide definitive evidence on the effect prophylactic dressings have on the development of hospital-acquired sacral pressure injuries in medical-surgical patients. A parallel economic evaluation of pressure injury prevention and treatment will enable evidence-informed decisions and policy. The inclusion of a process evaluation will help to explain the contextual factors that may have a bearing on trial results including the acceptability of the dressings to patients and staff. The trial commenced 5 March 2020 and has been significantly delayed due to COVID-19. Trial registration: ANZCTR ACTRN12619000763145. Prospectively registered on 22 May 2019