Postpartum anemia is associated with maternal and perinatal morbidity. Population-level data may inform guideline development for postpartum anemia screening. Our objectives were to evaluate the associations between potential predictors (predelivery anemia and postpartum hemorrhage [PPH]) with severe postpartum anemia after Cesarean section. Data were collected from 70,939 hospitalizations for Cesarean section performed at Kaiser Permanente Northern California facilities between 2005 and 2013. Severe postpartum anemia was defined as a hemoglobin (Hb) level of less than 8 g/dL before hospital discharge. Using multivariable logistic regression, we assessed the associations between predelivery anemia and PPH with severe postpartum anemia. Distributions of these characteristics among women with severe postpartum anemia were evaluated. The overall rate of severe postpartum anemia was 7.3% (95% confidence interval [CI], 7.1%-7.4%). Severe postpartum anemia was strongly associated with a predelivery Hb level between 10 and 10.9 g/dL (adjusted odds ratio [aOR], 5.4; 95% CI, 4.89-5.91), predelivery Hb level of less than 10 g/dL (aOR, 30.6; 95% CI, 27.21-34.6), and PPH (aOR, 8.45; 95% CI, 7.8-9.16). The proportions of women with severe postpartum anemia were highest for those experiencing PPH but no predelivery anemia (12.2%; 95% CI, 11.0%-13.6%) and those who did not incur PPH nor predelivery anemia (10.7%; 95% CI, 9.6%-12.0%). Our findings suggest that PPH and predelivery anemia are strong independent risk factors for severe postpartum anemia. Optimization of patients’ Hb before delivery may reduce the incidence of severe anemia after Cesarean section.