The chaga medicinal mushroom (Inonotus obliquus) was traditionally used to treat various ailments. To establish the pharmacological properties of I. obliquus, studies were performed to show the antiulcer activity of the ethanolic extract. The ethanolic extract of I. obliquus was prepared. The antiulcer activity of I. obliquus was determined using gastric ulcerated rats (ulceration induced by ethanol). The ethanolic extract of I. obliquus (200 mg/kg) did not cause any sign of toxicity or sensitivity to rats when the extracts were administered by oral feed. Oral administration of ethanolic extract of I. obliquus exhibited antiulcer activity in all models used. The ethanolic extract of I. obliquus showed an effective antiulcer activity, which could be due to the presence of various biologically active compounds. This confirmed the traditional uses of I. obliquus in the treatment of ailments.