An 85-year-old cachectic man was found unconscious in his home. He had no specific medical history. On arrival, he was in a deep coma and hypothermic state. He had a soft mass the size of his fist in the right lower abdomen without redness or heat. Truncal computed tomography revealed subcutaneous fluid collection with gas formation. A test puncture for right lower abdominal subcutaneous fluid collection revealed pus, so an open incision was performed, with the administration of broad-spectrum antibiotics. Unfortunately, the patient died of sepsis-induced multiple organ failure. The results of abscess culture later revealed Proteus mirabilis, Escherichia coli, and Prevotella melaninogenica. This is the first report of a cold abscess induced by mixed bacteria.