Background: Vitamin D deficiency is a prevalent condition among critically ill patients. Information about the relationship between vitamin D levels and outcomes in the intensive care unit (ICU) is sparse. Purpose: To evaluate vitamin D status among critically ill patients and its relevance to severity of illness, ICU stay period, and mortality. Methods: This prospective multicenter study was conducted in the ICUs of Fayoum, Cairo, Alazhar, and Ain Shams university hospitals. All patients were subjected to interview questionnaire, laboratory investigation, vitamin D level assessment, and severity of illness evaluation using the Acute Physiologic Assessment and Chronic Health Evaluation II (APACHE II) score. Results: In total, 250 patients were included in the study. The median age was 62 (40–73) years, and most patients were male (52%). The median serum level of vitamin D was 19 (7–40.6). Vitamin D was deficient in 197 patients (78.8%) on admission. While we grouped the ICU patients as vitamin D deficient, insufficient, and sufficient, vitamin D–deficient patients had more severe diseases (mean APACHE II score, 44 ± 15; P = .014). Prolonged ICU stay was observed among the deficient group but with no significant association. The overall mortality rate was 6.8%; of these, 70.5% were vitamin D–deficient patients. However, logistic regression analysis demonstrated that vitamin D deficiency was not an independent risk factor for mortality. Conclusion: Vitamin D insufficiency is common in critically ill patients (69%); it is associated with more severity of illness, but it is not an independent risk factor for longer ICU stay or mortality.