In this study, the phytoconstituents, acute toxicity and protective effects of ethanol extract of Dennettia tripetala (pepper fruit) seed against aspirin-induced ulcers in rats were evaluated using classical methods. For the antiulcer study, 20 albino wistar rats weighing between 120-250 g were acclimatized for 14 days and randomly assigned into 5 groups of 4 rats each. The rats were treated orally as follows: Group 1 (3 ml/kg 3% Tween 80), Groups 2, 3 and 4 (100, 200 and 400 mg/kg b.w. of ethanol extract of D. tripetala seed respectively) and Group 5 (100 mg/kg b.w. Omeprazole). Ulcerogenic lesions were induced using aspirin, and mean ulcer index, percentage inhibition of ulcer lesions and histological profiles were determined. Phytochemical analysis of D. tripetala seeds showed the presence of alkaloids, reducing sugars, cardiac glycosides, carbohydrates, flavonoids, tannins, oil and steroids in high concentration. Similarly, moderate amounts of saponins and resins, and low amounts of glycosides, acid and phlobatannins were detected. There was neither clinical sign nor mortality after 24 hours post-treatment observation in the rats, up to 5000 mg/kg b.w. of the extract in the acute toxicity study. Oral administration of ethanol extract of Dennettia tripetala seed at doses of 100, 200 and 400 mg/kg b.w. inhibited gastric ulceration in rats in a dose-dependent manner when compared to normal control. There was a strong positive correlation between % ulcer inhibition and concentration of extract administered (R2 = 0.997). Histological examination of the rat gastric tissues indicated that rats treated with extract and standard drug showed reduction in the ulcerations when compared with the control group. The results showed that ethanol extract of D. tripetala seed has potent and dose-dependent anti-ulcer effect against aspirin-induced ulcer which could be attributed to its flavonoid content.