We report a case of phlegmonous gastritis associated with Kaposi sarcoma in a 37-year-old, human immunodeficiency virus (HIV)-positive man who presented with an acute abdomen. Computed tomographic scan revealed free fluid in the abdominal cavity and a thickened gastric wall. A partial gastrectomy was performed. The resected portion of stomach had a hemorrhagic, necrotic thickened wall and showed extensive, acute suppurative inflammation, especially in the submucosa, with focal transmural involvement. Beneath an area of healing ulceration, a focus of Kaposi sarcoma was present. Group A beta-hemolytic streptococcus was grown from peritoneal fluid, and treatment with numerous antibiotics was initiated. After a difficult postoperative course that responded to 8 weeks of antibiotic therapy, the patient was medically stable and discharged from the hospital on antiretroviral therapy for HIV. Phlegmonous gastritis is a rare and rapidly progressive bacterial infection of the gastric wall. Kaposi sarcoma is one of the most common malignancies in HIV-positive patients, and gastric involvement is relatively common in those patients with systemic Kaposi sarcoma. To our knowledge, this is the first reported case of phlegmonous gastritis associated with Kaposi sarcoma, and it represents a rare survival following surgical and antibiotic therapy.