Nattō (納豆) is a traditional Japanese food made from whole soybeans that have been fermented with Bacillus subtilis var.

[3][4][5] Nattō is often considered an acquired taste because of its powerful smell, strong flavor, and sticky, slimy texture.

One theory is that nattō was codeveloped in multiple locations in the distant past, since it is simple to make with ingredients and tools commonly available in Japan since ancient times.

[12] One story about the origin of nattō attributes it to the samurai Minamoto no Yoshiie (1039–1106), who was on a campaign in northeastern Japan between 1086 and 1088 AD.

[13][14] Another story involves Prince Shotoku (574–622), who is said to have wrapped the leftovers of boiled soybeans in straw bags for his horse.

These are salted, fermented and aged whole soybean seasonings or condiments invented in China and spread throughout East Asia.

They are usually made from fermented soybeans and with an ample amount of salt; however, the ingredients and production methods differ in Japan.

[13][15] A wooden slip was excavated in Heijō-kyō, which had the Chinese character 豉 (chǐ; soybean) written on it.

[15][16] The excavation of the slip is considered an evidence to support the hypothesis that the invention of nattō was based on the Chinese douchi imported to Japan.

Home production was historically done using rice straw to maintain moisture and as insulation with placement in naturally warmer parts of the home or fermentation shed, but is now done with moist towels over glass pans or perforated plastic wrap on thermostat-controlled heating pads.

At-home fermentation machines are also available but are rarely marketed specifically for nattō, which is a less finicky culture capable of sustaining its own ideal temperature once it gets going.

Many countries around Asia also produce similar traditional soybean foods fermented with Bacillus bacteria, such as shuǐdòuchǐ (水豆豉) of China, cheonggukjang (청국장) of Korea, thua nao (ถั่วเน่า) of Thailand, kinema of Nepal and the Himalayan regions of West Bengal and Sikkim, tungrymbai of Meghalaya, hawaijaar of Manipur, bekang um of Mizoram, akhuni of Nagaland, and piak of Arunachal Pradesh, India.