Gastric/gastroesophageal junction cancer (G/GEJC) is associated with a poor prognosis. Only a minority of patients present with localized disease and meet criteria for curative therapy which includes surgery as well as chemoradiation whereas the majority present with locally advanced or unresectable disease and have limited treatment options. The recent developments of immunotherapy have changed the treatment landscape of G/GEJC. Immunotherapy is now approved for adjuvant therapy for resected G/GEJC as well as for first-line therapy for unresectable G/GEJC in the United States. These developments have been accompanied by the approvals of several predictive biomarkers for immunotherapy. Additionally, ongoing clinical trials are evaluating the efficacy and safety of novel treatment strategies using immunotherapy as the cornerstone. Here, we outline the development of immunotherapy in G/GEJC to date focusing on the landmark clinical trials leading to the approval of specific immunotherapy agents and predictive biomarkers and introduce the ongoing studies that are evaluating novel treatment strategies