Objective No studies have evaluated the diagnostic value of ischemia-modified albumin (IMA) for the early detection of sepsis/septic shock in patients presenting to the emergency department (ED). We aimed to assess the usefulness of IMA in diagnosing sepsis/septic shock in the ED. Methods This retrospective, observational study analyzed IMA, lactate, high sensitivity C-reactive protein, and procalcitonin levels measured within 1 hour of ED arrival. Patients with suspected infection meeting at least two systemic inflammatory response syndrome criteria were included and classified into the infection, sepsis, and septic shock groups using Sepsis-3 definitions. Areas under the receiver operating characteristic curves (AUCs) with 95% confidence intervals (CIs) and multivariate logistic regression were used to determine diagnostic performance. Results This study included 300 adult patients. The AUC (95% CI) of IMA levels (cut-off ≥85.5 U/mL vs. ≥87.5 U/mL) was higher for the diagnosis of sepsis than for that of septic shock (0.729 [0.667–0.791] vs. 0.681 [0.613–0.824]) and was higher than the AUC of procalcitonin levels (cut-off ≥1.58 ng/mL, 0.678 [0.613–0.742]) for the diagnosis of sepsis. When IMA and lactate levels were combined, the AUCs were 0.815 (0.762–0.867) and 0.806 (0.754–0.858) for the diagnosis of sepsis and septic shock, respectively. IMA levels independently predicted sepsis (odds ratio, 1.05; 95% CI, 1.00–1.09; P=0.029) and septic shock (odds ratio, 1.07; 95% CI, 1.02–1.11; P=0.002). Conclusion Our findings indicate that IMA levels are a useful biomarker for diagnosing sepsis/ septic shock early, and their combination with lactate levels can enhance the predictive power for early diagnosis of sepsis/septic shock in the ED.