Fifty-seven people have died and more than 18,000 have sought hospital treatment since July 29, when Japan was blanketed by an extreme and unrelenting heatwave.
The government announced the startling figures on Tuesday, revealing that the number of people hospitalized has tripled from the previous week.
Those aged 65 and older accounted for 54 percent of the total, and of the 18,347 people that sought treatment, 729 had severe symptoms that required more than three weeks of inpatient care.
Tokyo had the highest amount of people hospitalized with 1,857 patients, followed by Aichi and Saitama prefectures.