Background: Both perioperative chemotherapy and postoperative chemoradiotherapy improve survival in patients with resectable gastric cancer from Europe and North America. To our knowledge, these treatment strategies have not been investigated in a head to head comparison. We aimed to compare perioperative chemotherapy with preoperative chemotherapy and postoperative chemoradiotherapy in patients with resectable gastric adenocarcinoma. Methods: In this investigator-initiated, open-label, randomised phase 3 trial, we enrolled patients aged 18 years or older who had stage IB– IVA resectable gastric or gastro-oesophageal adenocarcinoma (as defined by the American Joint Committee on Cancer, sixth edition), with a WHO performance status of 0 or 1, and adequate cardiac, bone marrow, liver, and kidney function. Patients were enrolled from 56 hospitals in the Netherlands, Sweden, and Denmark, and were randomly assigned (1:1) with a computerised minimisation programme with a random element to either perioperative chemotherapy (chemotherapy group) or preoperative chemotherapy with postoperative chemoradiotherapy (chemoradiotherapy group). Randomisation was done before patients were given any preoperative chemotherapy treatment and was stratified by histological subtype, tumour localisation, and hospital. Patients and investigators were not masked to treatment allocation. Surgery consisted of a radical resection of the primary tumour and at least a D1+ lymph node dissection. Postoperative treatment started within 4–12 weeks after surgery. Chemotherapy consisted of three preoperative 21-day cycles and three postoperative cycles of intravenous epirubicin (50 mg/m2 on day 1), cisplatin (60 mg/m2 on day 1) or oxaliplatin (130 mg/m2 on day 1), and capecitabine (1000 mg/m2 orally as tablets twice daily for 14 days in combination with epirubicin and cisplatin, or 625 mg/m2 orally as tablets twice daily for 21 days in combination with epirubicin and oxaliplatin), received once every three weeks. Chemoradiotherapy consisted of 45 Gy in 25 fractions of 1·8 Gy, for 5 weeks, five daily fractions per week, combined with capecitabine (575 mg/m2 orally twice daily on radiotherapy days) and cisplatin (20 mg/m2 intravenously on day 1 of each 5 weeks of radiation treatment). The primary endpoint was overall survival, analysed by intention-to-treat. The CRITICS trial is registered at ClinicalTrials.gov, number NCT00407186; EudraCT, number 2006-004130-32; and CKTO, 2006-02. Findings: Between Jan 11, 2007, and April 17, 2015, 788 patients were enrolled and randomly assigned to chemotherapy (n=393) or chemoradiotherapy (n=395). After preoperative chemotherapy, 372 (95%) of 393 patients in the chemotherapy group and 369 (93%) of 395 patients in the chemoradiotherapy group proceeded to surgery, with a potentially curative resection done in 310 (79%) of 393 patients in the chemotherapy group and 326 (83%) of 395 in the chemoradiotherapy group. Postoperatively, 233 (59%) of 393 patients started chemotherapy and 245 (62%) of 395 started chemoradiotherapy. At a median follow-up of 61·4 months (IQR 43·3–82·8), median overall survival was 43 months (95% CI 31–57) in the chemotherapy group and 37 months (30–48) in the chemoradiotherapy group (hazard ratio from stratified analysis 1·01 (95% CI 0·84–1·22; p=0·90). After preoperative chemotherapy, in the total safety population of 781 patients (assessed together), there were 368 (47%) grade 3 adverse events; 130 (17%) grade 4 adverse events, and 13 (2%) deaths. Causes of death during preoperative treatment were diarrhoea (n=2), dihydropyrimidine deficiency (n=1), sudden death (n=1), cardiovascular events (n=8), and functional bowel obstruction (n=1). During postoperative treatment, grade 3 and 4 adverse events occurred in 113 (48%) and 22 (9%) of 233 patients in the chemotherapy group, respectively, and in 101 (41%) and ten (4%) of 245 patients in the chemoradiotherapy group, respectively. Non-febrile neutropenia occurred more frequently during postoperative chemotherapy (79 [34%] of 233) than during postoperative chemoradiotherapy (11 [4%] of 245). No deaths were observed during postoperative treatment. Interpretation: Postoperative chemoradiotherapy did not improve overall survival compared with postoperative chemotherapy in patients with resectable gastric cancer treated with adequate preoperative chemotherapy and surgery. In view of the poor postoperative patient compliance in both treatment groups, future studies should focus on optimising preoperative treatment strategies. Funding: Dutch Cancer Society, Dutch Colorectal Cancer Group, and Hoffmann-La Roche.