Background/Aims: Fexuprazan is a novel potassium-competitive acid blocker that could be of benefit to patients with gastric mucosal injury. The aim of this study was to assess the 2-week efficacy and safety of fexuprazan in patients with acute or chronic gastritis. Methods: In this study, 327 patients with acute or chronic gastritis who had one or more gastric erosions on endoscopy and subjective symptoms were randomized into three groups receiving fexuprazan 20 mg once a day (q.d.), fexuprazan 10 mg twice a day (b.i.d.), or placebo for 2 weeks. The posttreatment assessments were the primary endpoint (erosion improvement rate), secondary endpoints (cure rates of erosion and edema and improvement rates of redness, hemorrhage, and subjective symptoms), and drug-related adverse events. Results: Among the patients, 57.8% (59/102), 65.7% (67/102), and 40.6% (39/96) showed erosion improvement 2 weeks after receiving fexuprazan 20 mg q.d., fexuprazan 10 mg b.i.d., and placebo, respectively. Both fexuprazan 20 mg q.d. and 10 mg b.i.d. showed superior efficacy to the placebo (p=0.017 and p<0.001, respectively). Likewise, both fexuprazan 20 mg q.d. and 10 mg b.i.d. also showed higher erosion healing rates than the placebo (p=0.033 and p=0.010, respectively). No difference was noted in the edema healing rate and the improvement rates for redness, hemorrhage, and subjective symptoms between the fexuprazan and placebo groups. No significant difference was noted in the incidence of adverse drug reactions. Conclusions: Fexuprazan 20 mg q.d. and 10 mg b.i.d. for 2 weeks showed therapeutic efficacy superior to that of placebo in patients with acute or chronic gastritis (ClinicalTrials.gov identifier NCT04341454).