A 6-year-old Belgian gelding presented with a two month history of intermittent fever and colic despite several courses of antimicrobials. On presentation, the gelding had decreased borborygmi, fever, tachycardia, soft stool, and displayed signs of colic. Abdominal ultrasound revealed a large amount of peritoneal fluid and a possible abscess in the right caudal abdomen. Cytology of the fluid revealed a septic suppurative exudate. The owners elected conservative medical management, and the gelding was discharged on penicillin, enrofloxacin, and phenylbutazone. Despite treatment, the gelding presented 6 days later in septic shock. Euthanasia was elected, and a necropsy was performed. A diagnosis of septic peritonitis with a pericolonic abscess and an intralesional perforating metal foreign body was made at necropsy. Although far more common in cattle, perforating metal foreign bodies should be kept on the differential list for horses with unexplained peritonitis.