Background <p>Severely injured patients may suffer from acute disease-related or injury-related malnutrition involving a marked inflammatory response. This study investigated the prevalence and incidence of malnutrition and its relation with complications in severely injured patients admitted to the intensive care unit (ICU).</p> Methods <p>This observational prospective cohort study included severely injured patients (Injury Severity Score ≥ 16), admitted to the ICU of five level-1 trauma centers in the Netherlands and United States. Malnutrition was defined as a Subjective Global Assessment score ≤ 5. Complications included systemic-, surgery-, and fracture-related complications, pneumonia, urinary tract infection, deep venous thrombosis, and pulmonary embolism. In-ICU and in-hospital mortality were recorded separately. The complication rate was compared between patients who had or developed malnutrition and patients who remained well-nourished, using multivariable logistic regression analysis.</p> Results <p>Of 100 included patients, twelve (12%) were malnourished at admission. Of the 88 well-nourished patients, 44 developed malnutrition during ICU admission, (ICU incidence 50%, 95% confidence interval [CI] 40–60%). Another 18 patients developed malnutrition at the ward (overall in-hospital incidence 70%, 95% CI 61–80%). The 62 patients who developed malnutrition and 12 patients who were malnourished upon admission had more complications than the 26 patients who remained well-nourished (58% vs. 50% vs. 27% respectively; <i>p</i> = 0.03; Odds Ratio 3.4, 95% CI 1.2–9.6).</p> Conclusions <p>50% of severely injured patients developed malnutrition during ICU admission, increasing to 70% during hospital admission. Malnutrition was related to an increased risk of complications. Recognition of sub-optimally nourished severely injured patients and assessment of nutritional needs could be valuable in optimizing their clinical outcomes.</p> Level of evidence <p>Level III, Prognostic/Epidemiological.</p>