The aim of this study was to determine whether initial shock index (SI) was independently associated with the requirement for massive transfusion (MT) in emergency department (ED) patients with primary postpartum hemorrhage (PPH). A retrospective cohort study of ED patients with primary PPH was performed at a university-affiliated, tertiary referral center between January 2004 and May 2012. Patients were classified to two groups: MT group (patients who received ≥10 U of packed red blood cells within 24 h of ED admission) and non-MT group (patients who received <10 U). Variables of the two groups were compared using univariate and multivariate analyses. A total of 126 patients were included in this study. Of these patients, 26 (20.6%) were included in MT group and 100 (79.4%) in non-MT group. Patients in MT group had significantly lower blood pressure and higher heart rate compared with patients in non-MT group (P < 0.01). Initial SI was significantly higher in MT group than in non-MT group (1.3 vs 0.8, P < 0.01). In multivariate logistic regression analysis, initial SI and heart rate were the only variables associated with the requirement for MT, with an odds ratio of 9.47 (95% confidence interval, 1.75-51.28; P < 0.01) and 1.06 (95% confidence interval, 1.02-1.09; P < 0.01), respectively. In conclusion, initial SI was independently associated with the requirement for MT in ED patients with primary PPH. Routine calculation of initial SI can help clinicians to identify patients who may benefit from timely and appropriate use of MT to improve clinical outcomes.