BACKGROUND: Early mobilization has been proven effective for patients in intensive care units (ICUs) to improve functional recovery. However, early mobilization of critically ill, often mechanically ventilated, patients is cumbersome because of the attachment to tubes, drains, monitoring devices and muscle weakness. A mobile treadmill with bodyweight support may help to initiate mobilization earlier and more effectively. The aim of this study is to assess the effectiveness of weight-supported treadmill training in critically ill patients during and after ICU stay on time to independent functional ambulation. METHODS: In this randomized controlled trial, a custom-built bedside body weight-supported treadmill will be used and evaluated. Patients are included if they have been mechanically ventilated for at least 48 hours, are able to follow instructions, have quadriceps muscle strength of Medical Research Council sum-score 2 (MRC 2) or higher, can sit unsupported and meet the safety criteria for physical exercise. Exclusion criteria are language barriers, no prior walking ability, contraindications for physiotherapy or a neurological condition as reason for ICU admission. We aim to include 88 patients and randomize them into either the intervention or the control group. The intervention group will receive usual care plus bodyweight-supported treadmill training (BWSTT) daily. The BWSSTT consists of walking on a mobile treadmill while supported by a harness. The control group will receive usual care physiotherapy treatment daily consisting of progressive activities such as bed-cycling and active functional training exercises. In both groups, we will aim for a total of 40 minutes of physiotherapy treatment time every day in one or two sessions, as tolerated by the patient. The primary outcome is time to functional ambulation as measured in days, secondary outcomes include walking distance, muscle strength, status of functional mobility and symptoms of post-traumatic stress. All measurements will be done by assessors who are blinded to the intervention on the regular wards until hospital discharge. DISCUSSION: This will be the first study comparing the effects of BWSTT and conventional physiotherapy for critically ill patients during and after ICU stay. The results of this study contribute to a better understanding of the effectiveness of early physiotherapy interventions for critically ill patients. TRIAL REGISTRATION: Dutch Trial Register (NTR) ID: NL6766. Registered at 1 December 2017.