Objectives: Recent reviews demonstrated discordant effects of ICU-based physical rehabilitation on physical function. These inconsistencies may be related to differences in treatment fidelity-the extent to which a protocol is delivered as planned. Before evaluating the association of fidelity with outcomes, we must first understand the extent of treatment fidelity reporting in ICU-based physical rehabilitation randomized controlled trials (RCTs)., Data Sources: Six electronic databases from inception to December 2022., Study Selection: We included RCTs enrolling adults or children admitted to the ICU, if greater than or equal to 50% were invasively mechanically ventilated greater than 24 hours, and underwent an ICU-based physical rehabilitation intervention, with no limitation to comparators or outcomes., Data Extraction: We screened and extracted data independently and in duplicate, with a third reviewer as needed. Extracted data included study characteristics, treatment descriptions, and the presence of National Institutes of Health Behaviour Change Consortium (NIH-BCC) treatment fidelity tool components. Treatment fidelity scores were calculated as the proportion of reported (numerator) out of total NIH-BCC components (denominator). We calculated scores across studies and by treatment group (intervention vs. comparator). We used linear regression to assess for a time trend in study treatment fidelity scores., Data Synthesis: Of 20,433 citations, 94 studies met inclusion criteria. Authors reported a median (first-third quartiles) of 19% (14-26%) of treatment fidelity components across studies. Intervention group scores were higher than comparator groups (24% [19-33%] vs. 14% [5-24%], p < 0.01). We found a mean increase in study treatment fidelity scores by 0.7% (0.3 points) per year., Conclusions: Only 19% of treatment fidelity components were reported across studies, with comparator groups more poorly reported. Future research could investigate ways to optimize treatment fidelity reporting and determine characteristics associated with treatment fidelity conduct in ICU-based physical rehabilitation RCTs., Competing Interests: Dr. Kho held a Canada Research Chair in Critical Care Rehabilitation and Knowledge Translation from the Canadian Institutes of Health Research during this study. Mr. Farley received support for article research from the Canadian Institutes of Health Research. The remaining authors have disclosed that they do not have any potential conflicts of interest., (Copyright © 2024 by the Society of Critical Care Medicine and Wolters Kluwer Health, Inc. All Rights Reserved.)