The aim of the present study is to evaluate the in vivo gastro protective and antioxidant activity of the marine red algae Griffithsia Pacifica Kylin (GPK) against indomethacin (IND)-induced gastric ulcer on experimental rats. The results revealed by the ethanolic extract of GPK produced significant reduction in gastric mucosal lesions, malondialdehyde (MDA), and tumour necrosis factor - α (TNF-α) associated with a remarkable increase in gastric juice, mucin content and gastric mucosal catalase (CAT), Nitric oxide (NO), and Prostaglandin E2 (PGE2) levels. The volume and acidity of the gastric juice decreased in pretreated rats. The GPK algae extract was elevated in the gastric juice of rats untreated has showed near normal levels in pretreated rats. The GPK were able to decrease acidity and increase the mucosal defense in the gastric area. Ranitidine (RAN) significantly increased pH value and decreased pepsin activity and gastric juice free and total acidity. The antiulcer effect was further confirmed histologically. Finally the current study justifying the traditional usage of these GPK to treat gastric ulcers.