Background: The programmed death-1 receptor (PD-1) and its ligand (PD-L1) are key therapeutic targets in the reactivation of the immune response against cancer. Avelumab* (MSB0010718C) is a fully human anti-PD-L1 IgG1 antibody being investigated in clinical trials. The phase Ib study (NCT01772004) is an open-label, parallel-group expansion trial in patients (pts) with locally advanced or metastatic (LA/M) solid tumors that includes a heavily pretreated cohort of pts with advanced gastric or gastroesophageal junction adenocarcinoma (GC/GEJ) to evaluate safety and efficacy of avelumab in the 3^rd-line setting. Prior to adding this 3^rd-line cohort, this trial had enrolled a separate cohort with advanced GC/GEJ who had received prior 1^st-line chemotherapy (Chung et al, ECC 2015). Methods: This trial cohort is enrolling pts with histologically confirmed unresectable LA/M GC/GEJ who have been previously treated with 1^st-line combination chemotherapy and 2^nd-line ramucirumab, alone or in combination, and whose disease has progressed during or after ramucirumab treatment. Pts who have received prior treatment with trastuzumab and pts with HER2+ status are allowed. Eligible pts also must have tumor archival material or fresh biopsy, an ECOG performance status of 0 or 1 at the time of trial entry, and disease with ≥ 1 measurable lesion according to RECIST 1.1. Exclusions include prior therapy with immune checkpoint drugs or history of autoimmune disease. Up to 150 eligible pts will receive avelumab at 10 mg/kg as an IV infusion Q2W. Treatment will continue until disease progression, unacceptable toxicity, or any criterion for withdrawal occurs. Primary endpoint is confirmed best overall response (RECIST 1.1) as adjudicated by an IERC. Secondary objectives include assessment of progression-free survival, overall survival, and immune-related efficacy assessments. Association between tumor PD-L1 expression and efficacy will be evaluated. Adverse events will be assessed and graded according to NCI-CTCAE v4.0. Tumor evaluation will be performed every 6 wks until progression. Enrollment in this cohort began in June 2015. *Proposed INN Clinical trial information: NCT01772004