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Introduction Inflammation and infection, including dental infectious diseases, are factors that can induce preterm birth. We previously reported that mice with dental Porphyromonas gingivalis infection could be used as a model of preterm birth. In this model, cyclooxygenase (COX)-2 and interleukin (IL)-1β levels are increased, and P. gingivalis colonies are observed in the fetal membrane. However, the mechanism underlying fetal membrane inflammation remains unknown. Therefore, we investigated the immune responses of human amnion to P. gingivalis in vitro. Methods Epithelial and mesenchymal cells were isolated from human amnion using trypsin and collagenase, and primary cell cultures were obtained. Confluent cells were stimulated with P. gingivalis lipopolysaccharide (P.g-LPS) or P. gingivalis. mRNA expressions of IL-1β, IL-8, IL-6 and COX-2, protein expressions of nuclear factor (NF)-κB pathway components and culture medium levels of prostaglandin E2 were evaluated. Results Following stimulation with 1 μg/mL P.g-LPS, the mRNA expression levels of IL-1β, IL-8, IL-6 and COX-2 in mesenchymal cells were increased 5.9-, 3.3-, 4.2- and 3.1-fold, respectively. Similarly, the expression levels of IL-1β, IL-8, IL-6 and COX-2 in mesenchymal cells were increased by 7.6-, 8.2-, 13.4- and 9.3-fold, respectively, after coculture with P. gingivalis. Additionally, stimulation with P.g-LPS or P. gingivalis resulted in the activation of NF-κB signaling and increased production of IL-1β and prostaglandin E2. In contrast, no significant changes were observed in epithelial cells. Discussion Our findings suggest that mesenchymal cells might mediate the inflammatory responses to P. gingivalis and P.g-LPS, thereby producing inflammation that contributes to the induction of preterm birth.