Kokutai (国体; "national body/structure of state") is a concept in the Japanese language translatable as "system of government", "sovereignty", "national identity, essence and character", "national polity; body politic; national entity; basis for the Emperor's sovereignty; Japanese constitution" or nation.

Kokutai originated as a Sino-Japanese loanword from Chinese guoti (Chinese: 國體; pinyin: guótǐ; "state political system; national governmental structure"). The Japanese compound word joins koku (國; "country; nation; province; land") and tai (體; "body; substance; object; structure; form; style"). According to the Hanyu Da Cidian, the oldest guoti usages are in two Chinese classic texts. The 2nd century BC Guliang zhuan (榖梁傳; 'Guliang's Commentary') to the Spring and Autumn Annals glosses dafu (大夫; 'high minister', 'senior official') as guoti metaphorically meaning "embodiment of the country". The 1st century AD Book of Han history of Emperor Cheng of Han used guoti to mean "laws and governance" of Confucianist officials.

The historical origins of kokutai go back to pre-1868 periods, especially the Edo period ruled by the Tokugawa shogunate (1603–1868).

Aizawa Seishisai (会沢正志斎, 1782–1863) was an authority on Neo-Confucianism and leader of the Mitogaku (水戸学 "Mito School") that supported direct restoration of the Imperial House of Japan. He popularized the word kokutai in his 1825 Shinron (新論 "New Theses"), which also introduced the term Sonnō jōi ("revere the Emperor, expel the barbarians").

Aizawa developed his ideas of kokutai using the idea that the Japanese national myths in the Kojiki and Nihon Shoki were historical facts, believing that the Emperor was directly descended from the sun goddess Amaterasu-ōmikami. Aizawa idealized this divinely-ruled ancient Japan as a form of saisei itchi (祭政一致 "unity of religion and government") or theocracy. For early Japanese Neo-Confucian scholars, linguist Roy Andrew Miller says, "kokutai meant something still rather vague and ill defined. It was more or less the Japanese 'nation's body' or 'national structure'."

Katō Hiroyuki (1836–1916) and Fukuzawa Yukichi (1835–1901) were Meiji period scholars who analyzed the dominance of Western civilization and urged progress for the Japanese nation.

In 1874, Katō wrote the Kokutai Shinron (国体新論 "New Theory of the National Body/Structure"), which criticized traditional Chinese and Japanese theories of government and, adopting Western theories of natural rights, proposed a constitutional monarchy for Japan. He contrasted between kokutai and seitai (政体 "government body/structure"). Brownlee explains.

The Kokutai-seitai distinction enabled conservatives to identify clearly as Kokutai, National Essence, the "native Japanese", eternal, and immutable aspects of their polity, derived from history, tradition, and custom, and focused on the Emperor. The form of government, Seitai, a secondary concept, then consisted of the historical arrangements for the exercise of political authority. Seitai, the form of government, was historically contingent and changed through time. Japan had experienced in succession direct rule by the Emperors in ancient times, then the rule of the Fujiwara Regents, then seven hundred years of rule by shōguns, followed by the allegedly direct rule of the Emperors again after the Meiji Restoration. Each was a seitai, a form of government. In this understanding, the modern system of government under the Meiji Constitution, derived this time from foreign sources, was nothing more than another form of Japanese government, a new seitai. The Constitution was nothing fundamental. ^{[full citation needed]}