Zhu Xi (朱熹, Hanyu Pinyin: Zhū Xī, Wade-Giles: Chu Hsi) (1130 - 1200) was a Song Dynasty (960-1279) Confucian scholar who became one of most significant Neo-Confucians in China. He taught at the famous White Deer Grotto Academy for some time. During the Song Dynasty, Zhu Xi's teachings were considered to be unorthodox. Zhu Xi and his fellow scholars added three additional classics: the Four Books, the Great Learning, and the Doctrine of Mean to the Confucian canon. Their writings were not widely recognised in Zhu Xi's time; however, they later became accepted as standard commentaries on the Confucian classics.

Zhu Xi considered the earlier philosopher Xun Zi to be a heretic for departing from Confucius's beliefs about innate human goodness. Zhu Xi contributed to Confucian philosophy by articulating what was to become the orthodox Confucian interpretation of a number of beliefs in Daoism and Buddhism. He adapted some ideas from these competing religions into his form of Confucianism.

He argued that all things are brought into being by two universal elements discussed by Confucius and Mencius: vital (or physical) force (qi), and law or rational principle (li). Li is also called Tai Ji or Tai Chi, which means Great Ultimate. According to Zhu Xi, Tai Ji causes qi movement and change in the physical world, resulting in the division of the world into the two energy modes (yin and yang) and the five elements (fire, water, wood, metal, and earth).

According to Zhu Xi's theory, every physical object and every person contains aspects of li or Tai Ji. What is referred to as the human soul, mind, or spirit is defined as the Great Ultimate (Tai Ji), or the supreme regulative principle at work in a person. Zhu Xi argued that the fundamental nature of humans was morally good; even if people displayed immoral behaviour, the supreme regulative principle was good.

According to Zhu Xi, vital force (qi) and rational principle (li) operated together in mutual dependence. These are not entirely non-physical forces, but resulted in the creation of matter. When their activity is rapid the yang energy mode is generated, and when their activity is slow, the yin energy mode is generated. The yang and yin constantly interact, gaining and losing dominance over the other. This results in the structures of nature known as the five elements.

Zhu Xi discussed how he saw the Great Ultimate concept to be compatible with priciple of Daoism, but his concept of Tai Ji was different than the understanding of Dao in Daoism. Where Tai Ji is a differentiating principle that results in the emergence of something new, Dao was something that was still and silent and operated to reduce all things to equality and indistinquishability.

Zhu Xi's ideas also parted from Buddhism because he did not understand reality as a void without any attributes nor did he see the universe moving in that direction, as is found in Buddhism. Instead, he argued that there is a central harmony that is not static, empty but dynamic, and the Great Ultimate is in constant movement.

He did not hold to traditional ideas of God or Heaven (Tian), though he discussed how his own ideas mirrored the traditional concepts. He encouraged an agnostic tendency within Confucianism, because he believed that the Great Ultimate was a rational principle, and discussed it as an intelligent and ordering will behind the universe. He did not promote the worship of spirits and offerings to images. Although he practiced some forms of ancestor worship, he disagreed that the souls of ancestors existed, believing instead that ancestor worship is a form of rememberance and gratitude.

Zhu Xi practiced a form of daily meditation similar to, but not the same as, Buddhist dhyana or chan ding (ch'an-ting). His meditation did not require the cessation of fall thinking as in Buddhism, but was characterised by quiet introspection that helped to balance various aspects of one's personality and allowed for focused thought and concentration. His form of meditation was by nature Confucian in the sense that it was concerned with morality. His meditation attempted to reason and feel in harmony with the universe. He believed that this type of meditation brought humanity closer together in harmony.

The teachings of Zhu Xi were to dominate Confucianism, though dissenters would later emerge, such as Wang Yang-ming two and a half centuries later.

Life magazine ranked Zhu Xi as the 45th most important person in the last millennium.