Immunotherapy using anti-PD-1/PD-L1 antibodies for several types of cancer has received considerable attention in recent decades. However, the molecular mechanism underlying PD-L1 expression in pancreatic ductal adenocarcinoma (PDAC) cells has not been clearly elucidated. We investigated the clinical significance and regulatory mechanism of PD-L1 expression in PDAC cells. Among the various cytokines tested, tumor necrosis factor (TNF)-α upregulated PD-L1 expression in PDAC cells through NF-κB signaling. The induction of PD-L1 expression was also caused by co-culture with activated macrophages, and the upregulation was inhibited by neutralization with anti-TNF-α antibody after co-culture with activated macrophages. PD-L1 expression in PDAC cells was positively correlated with macrophage infiltration in tumor stroma of human PDAC tissues. In addition, survival analysis revealed that high PD-L1 expression was significantly associated with poor prognosis in 235 PDAC patients and especially in patients harboring high CD8-positive T-cell infiltration. These findings indicate that tumor-infiltrating macrophage-derived TNF-α could be a potential therapeutic target for PDAC.