Japanese sinologist, philosopher, and educator of the Edo (Tokugawa) period (1603–1867), who founded the Kogigaku (Kogaku) (“Study of Ancient Meaning”) school of thought , which subsequently became part of the larger Kogaku (“Ancient Learning”) school. Like his fellow Kogaku scholars, Yamaga Sokō and Ogyū Sorai, Itō came to oppose the official Neo-Confucianism of Tokugawa Japan—derived essentially from the writings of the Chinese thinker Zhu Xi—instead advocating a return to classical Confucian teaching. Through his hundreds of students, he exerted a powerful influence that tended to counteract the monolithic thought patterns imposed on the country by the Tokugawa rulers.

The son of a Kyōto lumberman, Jinsai turned his hereditary business over to his younger brother in order to devote himself to teaching and scholarship. He became known for his gentle manner and his dedication to humanistic ideals. Refusing all offers of employment from the powerful feudal rulers, he and his son Itō Tōgai (1670–1736) founded the Kogidō (“Hall of Ancient Meaning”) school in Kyōto. It was run by his descendants until 1904, when it was absorbed into the public school system.

The outline of Jinsai's thought, which is one of the most remarkable of the Tokugawa era for its level of moral elevation, can be found in a small work called Gōmōjigi (1683), a commentary on the writings of the Chinese philosophers Confucius and Mencius. Jinsai was concerned with what he saw as the underlying truths of Confucian thought. He tried to develop a rational, as against an authoritarian, basis for human morality and the pursuit of happiness.