Objective: To clarify whether isolated proteinuria (IP) is an independent risk factor for blood transfusion (BT) for postpartum hemorrhage (PPH), and whether risk factors for BT identified in single pregnancy also apply to twin pregnancy. Materials and Methods: A retrospective cohort study of consecutive women who gave birth at Jichi Medical University Hospital, Japan, between 1 April 2006 and 31 December 2016 was performed. Single or diamniotic twin deliveries producing healthy infants of ≥ 22 weeks were included. We analyzed the correlations between BT and 13 candidate risk factors that may be potentially associated with PPH in single and twin pregnancies. Results: We included 11,423 pregnancies: 10,523 (92.2%) single and 900 (7.8%) twin pregnancies. In single pregnancies, multivariate analysis indicated that placenta previa (PP), abruptio placentae, IP, chronic or gestational hypertension, preeclampsia (PE), HELLP syndrome, and tocolytic treatment (OR: 10.4, 19.1, 3.7, 3.6, 4.0, 18.9, and 2.0, respectively) were independent factors for the increased risk of allogenic BT. In twin pregnancies, multivariate analysis revealed that PP, abruptio placentae, IP, PE, and HELLP syndrome were independent factors for the increased risk of allogeneic BT (OR: 8.3, 103, 3.9, 4.3, and 39.6, respectively). Conclusion: IP was a novel risk factor for BT in both single and twin pregnancies. Although risk factors for BT were very similar between single and twin pregnancies, intravenous tocolysis was and was not a risk factor in single and twin pregnancies, respectively.