Gastric actinomycosis is an extremely rare disease. To date, about 20 cases have been reported in the literature. In most cases, diagnosis was made by histopathologic evaluation of an operative specimen. We report here a 68-year-old man with primary gastric actinomycosis who was admitted to the hospital with upper gastrointestinal bleeding and diagnosed as actinomycosis by microscopic examination of biopsy specimens obtained by endoscopy. This case is reported because of the rarity of endoscopically diagnosed primary gastric actinomycosis.