Introduction Muscle mass assessment of critically ill patients is essential to be part of the nutritional diagnosis in hospital care. Thus, the evaluation of more specific techniques for that purpose is needed. The present study aimed to investigate the association of quadriceps muscle thickness (QMT), measured by ultrasound (US), with the nutritional status of critically ill patients in a referral high-complexity trauma care hospital. Methods A cross-sectional observational study was conducted in the intensive care units (ICUs) in a tertiary hospital in Brazil. The sample comprised 30 critically ill trauma patients admitted between February and March 2022. The methodology involved evaluating muscle mass and comparing nutritional status through mid-upper arm circumference measurements and US assessments. Specifically, the QMT was quantified using US at a predefined site between the iliac crest and the proximal border of the patella. Results The Kruskal-Wallis test indicated variability in QMT between the nutritional status groups, with statistical significance reached after excluding the overweight group (H(2) = 7.532, p = 0.023). The moderate malnutrition group exhibited notably lower QMT. Sensitivity analyses using bootstrap and Monte Carlo methods showed moderate trends toward significance. A positive correlation was found between QMT and mid-upper arm circumference adequacy (p < 0.05), demonstrating fair to moderate correlation (rs = 0.531). Conclusion Significant changes in QMT were detected by ultrasound assessment in moderate malnutrition patients compared to patients of other nutritional statuses. Ultrasound may be a valuable technique for monitoring muscle integrity in critically ill patients.