Antithrombotic therapy is a major risk factor for delayed bleeding after endoscopic submucosal dissection (ESD) for gastric neoplasia. A potassium-competitive acid blocker, vonoprazan, is expected to prevent delayed bleeding better than conventional proton pomp inhibitors (PPIs), but the evidence is controversial. We sought to clarify the efficacy of vonoprazan for prevention of delayed bleeding after gastric ESD in patients under antithrombotic therapy. We prospectively registered 50 patients who underwent gastric ESD while receiving antithrombotic therapy and vonoprazan in our institution between October 2017 and September 2018. The incidence of delayed bleeding was compared with that in a historical control group of 116 patients treated with conventional PPI. We also evaluated risk factors associated with delayed bleeding. Delayed bleeding was observed in 8 of 50 patients (16.0%), which was not dissimilar from the incidence in the historical control group (12.1%) (p=0.49). In the univariate analysis, age (> 70 years) (p=0.034), multiple antithrombotic drug use (p<0.01), procedure time (> 200 min) (p=0.038) and tumor size (> 40 mm) (p<0.01) were associated with delayed bleeding after gastric ESD, but vonoprazan was not (p=0.49). Vonoprazan may not be more effective than conventional PPIs in preventing delayed bleeding after gastric ESD in patients receiving antithrombotic therapy.