introductionIn-bed leg cycling with critically ill patients is a promising intervention aimed at minimising immobility, thus improving physical function following intensive care unit (ICU) discharge. We previously completed a pilot randomised controlled trial (RCT) which supported the feasibility of a large RCT. In this report, we describe the protocol for an international, multicentre RCT to determine the effectiveness of early in-bed cycling versus routine physiotherapy (PT) in critically ill, mechanically ventilated adults. METHODS AND ANALYSIS: We report a parallel group RCT of 360 patients in 17 medical-surgical ICUs and three countries. We include adults (≥18 years old), who could ambulate independently before their critical illness (with or without a gait aid), ≤4 days of invasive mechanical ventilation and ≤7 days ICU length of stay, and an expected additional 2-day ICU stay, and who do not fulfil any of the exclusion criteria. After obtaining informed consent, patients are randomised using a web-based, centralised system to either 30 min of in-bed cycling in addition to routine PT, 5 days per week, up to 28 days maximum, or routine PT alone. The primary outcome is the Physical Function ICU Test-scored (PFIT-s) at 3 days post-ICU discharge measured by assessors blinded to treatment allocation. Participants, ICU clinicians and research coordinators are not blinded to group assignment. Our sample size estimate was based on the identification of a 1-point mean difference in PFIT-s between groups. ETHICS AND DISSEMINATION: TRIAL REGISTRATION NUMBER: NCT03471247 (Full RCT); NCT02377830 (CYCLE Vanguard 46 patient internal pilot).