This study was undertaken to evaluate the gastroprotective properties of seed, leaf, and branch methanolic extracts and guttiferone A obtained from Garcinia achachairu (Clusiaceae). Mice were used in all the models, and treatments were administered orally only in pylorus-ligated model of the extracts, and drugs were administered intraduodenally. Treatment with different extracts (500 mg/kg) significantly reduced the ulcerative lesions in the ethanol/HCl-induced model; however, the seed extract was most active. When tested in different doses (50, 250, or 500 mg/kg), the seed extract of G. achaicharu showed a dose-dependent effect with a percentage of inhibition of gastric lesions of 41, 49, and 85 %, respectively. The seed extract also significantly reduced the ulcerative lesions in the indomethacin/bethanechol-induced ulcer. In this model, the percentage of inhibition of ulcer was 24, 58, and 90 %, respectively. Regarding the model of gastric secretion, a reduction of gastric juice volume and total acidity was observed, as well as an increase in gastric pH. Considering that the seed extract was the most active, it was subjected to silica gel column chromatography, leading to the isolation of guttiferone A. The isolated compound and omeprazole were evaluated in the HCl/ethanol-induced ulcer model. In this assay, both compounds at a dose of 30 mg/kg reduced the ulcerative lesions by about 75 %. These results demonstrate, for the first time, that extracts obtained from G. achachairu and guttiferone A produce gastroprotective effects, corroborating ethnomedicinal use of this plant.