Senpai (先輩, "senior") and kōhai (後輩, "junior") are Japanese terms used to describe an informal hierarchical interpersonal relationship found in organizations, associations, clubs, businesses, and schools in Japan and expressions of Japanese culture worldwide. The senpai and kōhai relationship has its roots in Confucianism, but has developed a distinctive Japanese style. The term senpai can be considered a term in Japanese honorifics.

The relationship is an interdependent one, as a senpai requires a kōhai and vice versa, and establishes a bond determined by the date of entry into an organization. Senpai refers to the member of higher experience, hierarchy, level, or age in the organization who offers assistance, friendship, and counsel to a new or inexperienced member, known as the kōhai, who must demonstrate gratitude, respect, and occasionally personal loyalty. The kōhai defers to the senpai's seniority and experience, and speaks to the senpai using honorific language. The senpai acts at the same time as a friend. This relation is similar to the interpersonal relation between tutor and tutored in Eastern culture, but differs in that the senpai and kōhai must work in the same organization.

The relation originates in Confucian teaching, as well as the morals and ethics that have arrived in Japan from ancient China and have spread throughout various aspects of Japanese philosophy. The senpai–kōhai relation is a vertical hierarchy (like a father–son relation) that emphasizes respect for authority, for the chain of command, and for one's elders, eliminating all forms of internal competition and reinforcing the unity of the organization.

Over time this mechanism has allowed the transfer of experience and knowledge, as well as the expansion of acquaintances and the building of institutional memory. It also allows the development of beneficial experiences between both, as the kōhai benefits from the senpai's knowledge and the senpai learns new experiences from the kōhai by way of developing a sense of responsibility. This comradeship does not imply friendship; a senpai and kōhai may become friends, but such is not an expectation.

The Korean terms seonbae and hubae are written with the same Chinese characters and indicate a similar senior–junior relationship.^{[citation needed]} Both the Japanese and Korean terms are based on the Chinese honorifics xianbei (先輩/先辈) and houbei (後輩/后辈), written in the same Chinese characters.

Similar concept exists in the Chinese-speaking world, though the terms vary depending on the context. In business, the terms are usually qiánbèi (前輩/前辈) for seniors and hòubèi (後輩/后辈) for juniors. For students, the term is usually xuézhǎng/xuéjiě (學長/姐, more common in Taiwan) or shīxiōng/shījiě (师兄/姐, mainland China) for male and female senpai, respectively, and xuédì/xuémèi (學弟/妹, Taiwan) or shīdì/shīmèi (师弟/妹, mainland China) for male and female kohai, respectively. The student terms are also used in the Taiwanese military and the police system, though the existence of this seniority system in parallel to the ranks is criticized.

The senpai–kōhai system is deeply rooted in Japanese history. Three elements have had a significant impact on its development: Confucianism, the traditional Japanese family system, and the Civil Code of 1898.

Confucianism arrived from China between the 6th and 9th centuries, but the derived line of thought that brought about deep social changes in Japan was Neo-Confucianism, which became the official doctrine of the Tokugawa shogunate (1603–1867). The precepts of loyalty and filial piety as tribute (朝貢 chōkō) dominated the Japanese at the time, as respect for elders and ancestor worship that Chinese Confucianism taught were well accepted by the Japanese, and these influences have spread throughout daily life. Like other Chinese influences, the Japanese adopted these ideas selectively and in their own manner, so that the "loyalty" in Confucianism was taken as loyalty to a feudal lord or the Emperor.