Background: Patients (pts) with GC/GEC often present with A/M disease, which has a poor prognosis, with 1-year survival < 30%, and few treatment options. Nivolumab is a fully human anti-PD-1 IgG4 monoclonal antibody with a favorable safety profile and efficacy in melanoma, non–small-cell lung cancer, and renal cell carcinoma. The phase I/II, open-label CheckMate-032 study evaluated nivolumab ± ipilimumab in pts with solid tumors. Here, we report initial results for pts with GEC/GC receiving nivolumab monotherapy. Methods: Pts with A/M histologically confirmed GC/GEC, irrespective of PD-L1 status, were assigned to receive nivolumab alone (3 mg/kg IV Q2W) and treated until disease progression (PD) or intolerable toxicity. The primary endpoint was objective response rate (ORR); other endpoints included safety, progression-free survival, overall survival (OS), and biomarker status. Results: 59 pts were enrolled and treated with single-agent nivolumab. Median age was 60 y (range 29–80), and 83% of pts received ≥ 2 prior regimens. At database lock, 10 pts were on active treatment; 49 pts discontinued (PD, n = 40; unrelated adverse events, n = 4; treatment-related adverse events [TRAEs], n = 2; other, n = 3). Pts received a median of 4 doses (range 1–25). ORR was 12% (n = 7/58; 1 complete response, 6 partial responses); 12 pts (21%) had stable disease. Among responders, median duration of response was 7.1 mo (95% CI, 3.0–13.2). Median OS was 6.8 mo (95% CI, 3.3–12.4); 12-mo OS rate was 38% (95% CI, 23.2–52.7). 39% of tumor samples were PD-L1 positive ( ≥ 1% cutoff). ORRs in pts with PD-L1-positive and -negative tumors were 18% and 12%, respectively. TRAEs occurred in 66% of pts; most were Grade 1/2. Grade 3/4 TRAEs occurred in 14% of pts and included pneumonitis, fatigue, diarrhea, vomiting, hypothyroidism, and increased aspartate and alanine aminotransferase and alkaline phosphatase levels. No treatment-related deaths occurred. Conclusions: Nivolumab monotherapy was well tolerated and demonstrated encouraging antitumor activity in heavily pretreated pts with GC/GEC. Objective responses occurred in pts with PD-L1-positive and -negative tumors. Clinical trial information: NCT01928394