There currently are no internationally recognised treatment guidelines for patients with advanced gastric cancer/gastro-oesophageal junction cancer (GC/GEJC) in whom two prior lines of therapy have failed. The randomised, phase III JAVELIN Gastric 300 trial compared avelumab versus physician’s choice of chemotherapy as third-line therapy in patients with advanced GC/GEJC. Patients with unresectable, recurrent, locally advanced, or metastatic GC/GEJC were recruited at 147 sites globally. All patients were randomised to receive either avelumab 10mg/kg by intravenous infusion every 2weeks or physician’s choice of chemotherapy (paclitaxel 80mg/m2 on days 1, 8, and 15 or irinotecan 150mg/m2 on days 1 and 15, each of a 4-week treatment cycle); patients ineligible for chemotherapy received best supportive care. The primary end point was overall survival (OS). Secondary end points included progression-free survival (PFS), objective response rate (ORR), and safety. A total of 371 patients were randomised. The trial did not meet its primary end point of improving OS {median, 4.6 versus 5.0months; hazard ratio (HR)=1.1 95% confidence interval (CI) 0.9–1.4; P=0.81} or the secondary end points of PFS median, 1.4 versus 2.7months; HR=1.73 (95% CI 1.4–2.2); P>0.99 or ORR (2.2% versus 4.3%) in the avelumab versus chemotherapy arms, respectively. Treatment-related adverse events (TRAEs) of any grade occurred in 90 patients (48.9%) and 131 patients (74.0%) in the avelumab and chemotherapy arms, respectively. Grade ≥3 TRAEs occurred in 17 patients (9.2%) in the avelumab arm and in 56 patients (31.6%) in the chemotherapy arm. Treatment of patients with GC/GEJC with single-agent avelumab in the third-line setting did not result in an improvement in OS or PFS compared with chemotherapy. Avelumab showed a more manageable safety profile than chemotherapy. ClinicalTrials.gov: NCT02625623.