Human epidermal growth factor receptor 2 (HER2, also known as ERBB2) amplification or overexpression occurs in approximately 20% of advanced gastric or gastro-oesophageal junction adenocarcinomas. More than a decade ago, combination therapy with the anti-HER2 antibody trastuzumab and chemotherapy became the standard first-line treatment for patients with these types of tumours. Although adding the anti-programmed death 1 (PD-1) antibody pembrolizumab to chemotherapy does not significantly improve efficacy in advanced HER2-negative gastric cancer, there are preclinical and clinical rationales for adding pembrolizumab in HER2-positive disease. Here we describe results of the protocol-specified first interim analysis of the randomized, double-blind, placebo-controlled phase III KEYNOTE-811 study of pembrolizumab plus trastuzumab and chemotherapy for unresectable or metastatic, HER2-positive gastric or gastro-oesophageal junction adenocarcinoma ( https://clinicaltrials.gov , NCT03615326). We show that adding pembrolizumab to trastuzumab and chemotherapy markedly reduces tumour size, induces complete responses in some participants, and significantly improves objective response rate.