Syphilitic proctitis is a rare disease that usually presents as proctitis, ulcer, and neoplasm but lacks pathognomonic clinical symptoms. It is thus difficult to diagnose and may be treated inappropriately. We report a 31-year-old man who had a hard, ulcerated mass that occupied the rectal and sigmoid colon wall and mimicked a tumor. Fortunately, a biopsy of the mass demonstrated Treponema pallidum organisms consistent with syphilitic proctitis. The patient was successfully treated with intravenous benzyl penicillin, resulting in improvement in his proctitis.