Yamazaki Ansai (山崎 闇斎; Japanese pronunciation: [ja.maꜜ.(d)za.kʲi | aꜜɰ̃.sai], January 24, 1619 – September 16, 1682) was a Japanese philosopher and scholar. He began his career as a Buddhist monk, but eventually came to follow the teachings of Neo-Confucian Zhu Xi. He combined Neo-Confucian ideas with Shinto to create Suika Shinto.

Born in Kyoto on January 24, 1619, Yamazaki Ansai was the son of a former rōnin-turned-doctor and the last of four children. In his youth, he was strongly influenced by both his mother and grandmother. While his mother "urged him to develop a noble heart worthy of a samurai's son," his grandmother supported him in his study of the Chinese language. In his preteens, he was sent by his father to serve as an acolyte at a Buddhist temple on Mount Hiei. In his early teens, Ansai returned home, and after several years was finally permitted to enter the Myōshin-ji temple of the Rinzai Zen sect in Kyoto for further study. Due to his incredible scholarly aptitude, in his early twenties he was granted entrance to the Gyūkō-ji temple in Tosa. During his time at Tosa, he was strongly advised by his fellow monks to concentrate his studies on the teachings of Neo-Confucian scholars, thereby beginning the process of Ansai's conversion to Neo-Confucianism and ultimate rejection of Buddhism. Ansai was particularly captivated by the writings of the Song dynasty scholar, Zhu Xi (Chu Hsi), which later became the basis of Ansai's moral philosophy/teachings. At twenty eight, he returned to Kyoto, and under the patronage of Nonaka Kenzan, was able to continue his Neo Confucian studies, as well as begin to publish his own materials. With the production of his first work Heresies Refuted (Heikii, 1647), an outright rejection of Buddhist faith, Ansai fully embraced "the One True Way" of Neo Confucianism.

After his first publication, Ansai spent the remaining thirty-five years of his life writing, publishing, editing, annotating, and punctuating Confucian and Shinto texts (that accumulated to over two thousand pages). The decade following Tosa (1647–1657), Ansai lived, studied, and taught in Kyoto. There, he edited and published a great number of texts (mostly commentaries on the works of Chu Hsi). Ansai also frequently went to Edo, to give lectures on Cheng-Zhu school of Neo-Confucianism in front of a large number of daimyōs. In 1655, he established a private school in Kyoto, began his first lecture cycle in the spring of the same year, and finished it at the end of 1656.

Ansai's group of Confucian disciples was collectively referred to as the Kimon school. His lectures focused on Ansai's own, hand selected canon. His canon consisted mostly of the classic Confucian writings that Zhu Xi had emphasized: the Elementary Learning, the Reflections on Things at Hand, and the Four Books (the Great Learning, the Doctrine of the Mean, the Analects of Confucius, and the Mencius). However, he also included Cheng Yi's Commentary on the Book of Changes. In the 1660s and 1670s, Ansai (following the example of Zhu Xi) personally edited the six books that comprised his canon.

As a teacher, Ansai was described by his students as "extremely strict, sometimes scary, and short tempered." Generally speaking, Ansai had a reputation for being "single minded, doctrinate, and intolerant." Kaibara Ekken, a contemporary of Ansai's, had attended several of his lectures, and found Ansai to be: "severe, dogmatic, and more interested in strict moral discipline than in investigation of the principles for practical learning." Such prominent Neo-Confucian scholars as Kinoshita Jun'an, Asami Keisai, Miyake Shōsai, and Satō Naokata were included amongst Ansai's followers of the Kimon school.

In 1658, Ansai moved to Edo, where he spent the next 7 years of his life, continuing his studying of Neo-Confucian texts, as well as beginning his research on a never completed historiography of Japan (based upon Shinto texts). In 1665, after building up a reputation in both Edo and Kyoto as an extraordinary teacher, was invited by Hoshina Masayuki (the daimyō of Aizu) to become his teacher. Ansai accepted the position and spent the next seven years of his life as the personal teacher of Neo-Confucian philosophy to Masayuki. For tutoring Masayuki for six out of the twelve months of the year, Ansai was given a salary of 100 gold ryō, two seasonal garments, and one haori coat.

Although Ansai and Masayuki were said to share a close relationship, Ansai refused to become his vassal, declaring that Confucian scholars should remain autonomous of another individual's influence. Masayuki proved to be Ansai's intellectual equal, helping him compile five different works: two gazetteers for the Aizu domain, and three Confucian texts: Gyokusan kōgi furoku (Appendix to Zhu Xi's lecture at Yushan), Nitei jikyōroku (Record of the two Cheng's political teachings), and the Irakusanshiden shinroku (Record of the mind-heart). During his years of service to Masayuki, Ansai compiled more writings of Zhu Xi during his off time in Kyoto. These included: Jinsetsumondō (Questions and answers on explanations of "humaneness"), Shōgaku mōyōshu and Daigaku keihatsu shū (Collections of [clarifications by Zhu Xi] on the Elementary Learning and the Great Learning).

Due to this relationship, Ansai is considered to be one of the scholars most closely associated with the Tokugawa Bakufu. Also, Ansai was able to receive the secret teachings of the Yoshida and Ise Shinto traditions, which he would use in attempting to reconstruct a "pure Shinto", that would reflect the Way of Neo-Confucianism.