Gallarhois (GR) is a traditional oriental herbal medicine with various pharmacological effects; however, its effect on gastric ulcer has not been previously explored. We firstly investigated the component and antioxidant activity of GR extract (EtGR) by HPLC analysis and 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. The results showed that EtGR consisted of gallotannin (68.7%), gallic acid (27.2%) and methyl gallate (4.1%) and that it had a high antioxidant value (IC50 value; 1.93 μg/mL). To evaluate the possible anti-gastric ulcer potential of EtGR, we investigated the effects of EtGR in the model of ethanol/hydrochloric acid (EtOH/HCl)-induced gastric ulcer. Gross and histological gastric lesions, biochemical and gene expression parameters were taken into consideration. The results showed that EtOH/HCl treatment produced mucosal injuries with morphological and histological damage, whereas EtGR co-treatment reduced the gastric injuries. EtGR treatment also decreased the contents of malonaldehyde (MDA) activity relative to the vehicle group. Moreover, EtGR decreased the levels of interleukin-1β (IL-1β), interleukin-6 (IL-6) and cyclo-oxygenase-2 (COX-2) expression. Finally, EtGR did not induce any specific toxicity in the livers or kidneys of the EtOH/HCl-induced gastric ulcer model. These results suggest that EtGR had stronger antioxidant activity and could be a new useful natural drug for gastroprotection against gastric ulcer. Moreover, these findings provide a scientific basis for the development of drugs from traditional oriental herbal medicines.