Syphilitic proctitis is a rare disease. It usually presents as proctitis, ulcer and neoplasm but lacks pathognomonic clinical symptoms. It is, therefore, difficult to diagnose and is occasionally treated inappropriately. We report the case of a 51-year-old man who had a hard, ulcerated mass, which occupied the circumference of the rectal wall and which mimicked a rectal tumor. Fortunately, positive finding from routine toluidine red unheated serum test and treponema pallidum particle agglutination tests made us reevaluate the patient and led us to suspect syphilitic proctitis. This diagnosis was finally confirmed after successful penicillin G benzathine therapy which made surgery unnecessary.