Background: Despite improvements of postoperative adjuvant therapy for resected pancreatic ductal adenocarcinoma (PDAC), its prognosis remains poor. A randomized controlled trial has begun to compare neoadjuvant chemotherapy using gemcitabine and S1 (NAC-GS) with upfront surgery (Up-S) for patients with PDAC planned resection. Methods: Patients were enrolled after the diagnosis of resectable PDAC with histological confirmation. They were randomly assigned as either NAC-GS or Up-S. In NAC-GS, gemcitabine was provided at a dose of 1 g/m² on day 1 and 8 and oral S-1 was administered at a dose of 40 mg/m² twice daily on 1-14 days. Patients received 2 cycles of this regimen. S-1 adjuvant for 6 months was administered for the patients with curative resection and fully recovered within 10 weeks after surgery in both arms. The primary endpoint for the phase III part was overall survival (OS); secondary endpoints included adverse events, resection rate, recurrence-free survival, residual tumor status, nodal metastases, and tumor marker kinetics. The target sample size required 163 patients (α-error 0.05; power 0.8) in each arm. The trial was conducted by the Health Labor Sciences Research Grant (H22-009) of Japan and registered with the UMIN Clinical Trials Registry as UMIN000009634. Results: From January 2013 to January 2016, 364 patients were enrolled in 57 centers (182 to NAC-GS and 182 to Up-S). Of these, two were excluded because of ineligibility, therefore 182 patients in NAC-GS and 180 in Up-S constituted the ITT analysis-set. The median OS was 36.7 months in NAC-GS and 26.6 months in Up-S; HR 0.72 (95% confidential interval 0.55-0.94; p=0.015 [stratified log-rank test]). Grade 3 or 4 adverse events frequently (72.8%) observed in NAC-GS were leukopenia or neutropenia. However, the resection rate, R0 resection rate, and morbidity of the operation were equivalent in the two groups. There was no perioperative mortality in either group. Conclusions: This phase III study demonstrated the significant survival benefits of NAC-GS treatment. Therefore, the results indicated that neoadjuvant chemotherapy could be a new standard for patients with resectable PDAC. Clinical trial information: UMIN000009634.