A 21-year-old man with systemic lupus erythematosus (SLE) who developed acute lupus peritonitis is described. Acute lupus peritonitis appeared during generalized lupus flare, with nausea, vomiting, frequent diarrhea, and abdominal tenderness with rebound and guarding. The patient was afebrile and had decreased bowel sounds. Abdominal ultrasonography and computed tomography revealed marked thickening of the gastric, duodenal, and jejunal walls, massive intraluminal fluid collection, and increasing ascites. Gastrointestinal endoscopy showed edematous mucosa with multiple erosions of the stomach and duodenum. The ascitic fluid was remarkable for low complement levels and elevated anti-DNA antibody. These manifestations of acute lupus peritonitis resolved after steroid pulse therapy with methylprednisolone. We should consider acute lupus peritonitis in a patient with SLE when abdominal symptoms are severe. Experience with our patient indicates that steroid pulse therapy is effective for this rare but severe manifestation of SLE.