Background <p>Hypocalcemia is a common metabolic disturbance in trauma and may contribute to coagulopathy, hemodynamic instability, and adverse outcomes. This study aimed to investigate whether admission ionized calcium (iCa) levels are associated with in-hospital mortality and transfusion requirements in moderate-to-severe trauma patients.</p> Methods <p>This retrospective cohort study included adults (≥ 18 years) with moderate-to-severe trauma, defined as an Injury Severity Score (ISS) ≥ 9, admitted to the emergency department of a tertiary university hospital between January 2019 and December 2024. Patients with cardiac arrest, pregnancy, insufficient admission laboratory results, ISS &lt; 9, or pre-hospital transfusion were excluded. Severe hypocalcemia was defined as iCa &lt; 0.9 mmol/L. Baseline characteristics were compared between survivors and non-survivors, and the diagnostic performance of severe hypocalcemia (iCa &lt; 0.9 mmol/L) for predicting mortality and transfusion was analyzed.</p> Results <p>A total of 1,071 trauma patients were included; 136 (12.7%) died. Severe hypocalcemia was more common among non-survivors than survivors (23.5% vs. 7.3%; <i>p</i> &lt; 0.001) and among transfused patients than non-transfused patients (16.2% vs. 8.3%; <i>p</i> = 0.003). Severe hypocalcemia showed high specificity but low sensitivity for both mortality and transfusion prediction. In multivariate analysis, age, male gender, prolonged PT, pH &lt; 7.30, elevated lactate, and severe hypocalcemia were independent predictors of mortality.</p> Conclusion <p>Severe hypocalcemia is a significant predictor of adverse outcomes in trauma patients, remaining significant after adjustment for demographic and metabolic confounders. Future prospective studies are needed to determine whether early correction of hypocalcemia can improve survival and transfusion outcomes in trauma care.</p>