Background: Gastric Cancer (GC) is the third most common cause of cancer related deaths worldwide. The median overall survival of patients with stage 4 disease is approximately 1 year. Current accepted treatment approach with chemotherapy is applied with little consideration for known genetic or biologic heterogeneity. Whilst immune-based approaches in GC look promising it is clear that single-agent PD1/PDL1 inhibition benefit a minority. We must clarify a means of identifying prospectively those patients who may benefit from this treatment. A recent landmark paper by The Cancer Genome Atlas (TCGA) proposed a classification of GC into four subtypes: Epstein-Barr-virus (EBV)-positive, microsatellite instable (MSI), chromosomal instable (CI), and genomically stable (GS). Two of the four – EBV and MSI subtypes – are likely to be immunogenic and amenable to PD1/PDL1 inhibition. Recent advances have shown EBV-positive tumors to be infiltrated by lymphocytes and be enriched for PDL1. Methods: This single centre single-arm pilot study in gastric or junctional adenocarcinoma will explore the hypothesis that administering anti-PDL1 therapy (Avelumab) in a prospectively identified population enriched for potential responders will result in improved outcomes. The anticipated frequency of EBV associated-GC (c10%) means that approximately N = 100 patients will be screened to identify N = 10 participants. If a positive signal for efficacy is seen this will provide a basis for a larger, multicentre study. Previously treated Patients with confirmation of stage 4 EBV- positive gastric or oesophago-gastric adenocarcinoma meeting eligibility criteria will be enrolled. Avelumab will be administered at a dose of 10mg/kg IV every 14days. Primary endpoint is to determine the 6-month progression free survival (PFS) of Avelumab in EBV-associated GC. Secondary endpoints include overall response rate, overall survival, median PFS time and feasibility/accrual rate at 12 months. Exploratory endpoints will be to evaluate changes in immune parameters in the peripheral blood over time. Kaplan-Meier methods for primary efficacy endpoint with two-tailed one-sample proportion test will be used to evaluate the evidence to reject the null hypothesis. Clinical trial information: 2018-002085-39.