Intrauterine infection is the most commonly identified cause of preterm birth. In this study, our aim was to determine the association between placental inflammation and neonatal outcome in a prospective observational cohort of preterm infants of less than 34 weeks gestational age. We especially focused on the distinct effects of maternal inflammatory response (MIR) with and without fetal inflammatory response (FIR). Methods: Clinical characteristics and placental histological results were prospectively collected from 216 singleton infants born at a gestational age of less than 34 weeks. Results: Of the 216 newborns, 104 (48.1%) infants had histological placental inflammation. Based on their pathological findings, the premature infants were divided into three groups: (1) the MIR negative−FIR negative (MIR−FIR−) group; (2) the MIR positive−FIR positive (MIR+FIR+) group; and (3) the MIR positive–FIR negative (MIR+FIR−) group. The incidence of neonatal respiratory distress syndrome (RDS) in the MIR+FIR− group (5.7%) and in the MIR+FIR+ group (2.0%) was significantly lower than in the MIR−FIR− group (19.6%) (p < 0.05). Logistic regression analysis showed that MIR+FIR+ group had a decreased incidence of neonatal RDS (OR = 0.076; 95% CI 0.009–0.624; p = 0.016). The incidence of intraventricular hemorrhage (IVH) Grade 2 or greater was significantly higher in the MIR+FIR+ group (42.3%) than in the MIR+FIR− group (13.0%) (p < 0.05) or in the MIR−FIR− group (15.2%) (p < 0.05). Logistic regression analysis also showed that MIR+FIR+ was associated with an increased incidence of IVH Grade 2 or greater (OR = 4.08; 95% CI 1.259–13.24; p = 0.019). Conclusion: A positive MIR in association with a positive FIR decreases the risk of RDS, but increases the risk of IVH Grade 2 or greater in preterm infants with a gestational age of less than 34 weeks. However, a positive MIR alone has little effect on neonatal outcome.