Predicting the prognosis of unresectable pancreatic ductal adenocarcinoma (PDAC) is useful in determining the appropriate management strategy. The present study aimed to investigate the association between PDAC prognosis and inflammation-based markers, such as the neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio, prognostic nutritional index, modified Glasgow prognostic score (mGPS) and controlling nutritional status score. A total of 72 patients with unresectable PDAC who received chemotherapy were included. Inflammation-based markers were measured prior to treatment. The median progression-free survival (PFS) and overall survival (OS) were 117 days (range, 10-781 days) and 244 days (range 43-781 days), respectively. The cut-off value of continuous variables that predicted the median OS (244 days) was calcualted. Univariate analysis of PFS showed that disease stage, first-line chemotherapy regimen, carcinoembryonic antigen (CEA), NLR, platelet-to-lymphocyte ratio (PLR), mGPS and controlling nutritional status (CONUT) scores were associated with PFS. Among them, stage, first-line chemotherapy regimen, CEA, NLR and mGPS were independent prognostic factors for PFS in multivariate analysis. Univariate analysis of OS showed that stage, first-line chemotherapy regimen, CA19-9, NLR, PLR, prognostic nutritional index (PNI), mGPS and CONUT score were associated wtih OS. Among them, first-line chemotherapy and mGPS were independent prognostic factors for OS according to multivariate analysis. Univariate and multivariate analyses revealed that a NLR ≥4.0 and mGPS 2 were independent prognostic factors for PFS. For OS, mGPS 2 was an independent prognostic factor. In conclusion, mGPS was the most useful marker in predicting the prognosis of patients with unresectable PDAC who received chemotherapy.