Background Data on immune checkpoint inhibitor efficacy in patients with human epidermal growth factor receptor 2-positive (HER2+) advanced gastric/gastroesophageal junction (G/GEJ) cancer are lacking. Because HER2 status was not captured in the ATTRACTION-2 trial, we used patients with prior trastuzumab use (Tmab+) as surrogate for HER2 expression status to evaluate the efficacy and safety of nivolumab as third- or later-line therapy in these patients. Methods In ATTRACTION-2, a randomized, double-blind, placebo-controlled, phase 3 multicenter trial, patients were randomized (2:1) to receive nivolumab (3 mg/kg) or placebo every 2 weeks until disease progression or toxicity requiring study discontinuation. Overall survival (OS), progression-free survival (PFS), objective response rate (ORR), and safety were assessed. Results Of 493 enrolled patients, 81 (nivolumab, n  = 59; placebo, n  = 22) were Tmab+ and 412 (nivolumab, n  = 271; placebo, n  = 141) were Tmab−. In both groups, patients receiving nivolumab showed a longer median OS vs placebo (Tmab+, 8.3 95% confidence interval, 5.3–12.9 vs 3.1 1.9–5.3 months, hazard ratio, 0.38 0.22–0.66; P  = 0.0006; Tmab−, 4.8 4.1–6.0 vs 4.2 3.6–4.9 months, 0.71 0.57–0.88; P  = 0.0022). PFS was longer in both groups receiving nivolumab vs placebo (Tmab+, 1.6 1.5–4.0 vs 1.5 1.3–2.9 months, 0.49 0.29–0.85; P  = 0.0111; Tmab−, 1.6 1.5–2.4 vs 1.5 1.5–1.5 months, 0.64 0.51–0.80; P  = 0.0001). Conclusions Nivolumab was efficacious and safe as third- or later-line therapy regardless of prior trastuzumab use in patients with advanced G/GEJ cancer.