Hericium erinaceus is a famous tonic in oriental medicine. The gastroprotective effects of aqueous extract of H. erinaceus against ethanol-induced ulcers in Sprague Dawley rats were investigated. The possible involvements of lipid peroxidation, superoxide dismutase, and catalase were also investigated. Acute toxicity study was performed. The effects of aqueous extract of H. erinaceus on the ulcer areas, ulcer inhibition, gastric wall mucus, gross and histological gastric lesions, antioxidant levels, and malondialdehyde (MDA) contents were evaluated in ethanol-induced ulcer in vivo. In acute toxicity study, a high dose of 5 g/kg did not manifest any toxicological signs in rats. The extract promoted ulcer protection as ascertained by a significant reduction of the ulcer area. Furthermore, it exhibited a significant protection activity against gastric mucosal injury by preventing the depletion of antioxidant enzymes. The level of MDA was also limited in rat stomach tissues when compared with the ulcer control group. Immunohistochemistry showed upregulation of HSP70 protein and downregulation of BAX protein in rats pretreated with the extract. The aqueous extract of H. erinaceus protected gastric mucosa in our in vivo model. It is speculated that the bioactive compounds present in the extract may play a major role in gastroprotective activity.