Metformin-associated lactic acidosis (MALA) is a rare but highly fatal adverse event associated with metformin.We present a case of MALA in a patient with an unknown background, which precluded early diagnosis. A 59-year-old man was transported with complaints of epigastric pain and vomiting since the previous day. Upon arrival, the patient exhibited circulatory failure and altered consciousness, along with significant lactic acidosis. Although the cause was unclear, the patient was successfully treated with prompt intensive care including mechanical ventilation, administration of vasopressors and sodium bicarbonate, and continuous hemodiafiltration. Extubation was performed on the second day; the patient was able to walk and was discharged on the seventh day. Post-extubation interview revealed that the patient had taken metformin on the day before transport. Accordingly, we diagnosed the patient with MALA. Early diagnosis of MALA in severely ill patients with unknown medication history is challenging. However, in patients with severe unexplained lactic acidosis, a detailed medical history should be obtained, and MALA should be considered as a differential diagnosis with early intervention considering its fatal nature.