The Imperial House (皇室, Kōshitsu) is the reigning dynasty of Japan, consisting of those members of the extended family of the reigning emperor of Japan who undertake official and public duties. Under the present constitution of Japan, the emperor is "the symbol of the State and of the unity of the people". Other members of the imperial family perform ceremonial and social duties, but have no role in the affairs of government. The duties as an emperor are passed down the line to their male children. The Japanese monarchy is the oldest continuous hereditary monarchy in the world. The imperial dynasty does not have a name, therefore its direct members do not have a family name.

The imperial house recognizes 126 monarchs, beginning with Emperor Jimmu (traditionally dated to 11 February 660 BCE), and continuing up to the current emperor, Naruhito. However, scholars have agreed that there is no evidence of Jimmu's existence, that the traditional narrative of the imperial family's founding is mythical, and that Jimmu is a mythical figure. Historical evidence for the first 25 emperors is scant, and they are considered mythical, but there is sufficient evidence of an unbroken agnatic line since the early 6th century. Historically, verifiable emperors of Japan start from 539 CE with Emperor Kinmei, the 29th tennō.

The earliest historic written mentions of Japan were in Chinese records, where it was referred to as Wa (倭 later 和), which later evolved into the Japanese name of Wakoku. Suishō (ca. 107 CE) was a king of Wa, the earliest Japanese monarch mentioned in Volume 85 of the Book of the Later Han from 445 CE. Further records mention the five kings of Wa, of which the last one Bu of Wa is generally considered to be Emperor Yūryaku (417/18 – 479 CE). The existence of his reign has been established through modern archaeological research.

While the main line of the dynasty does not have a name and is referred to as Kōshitsu (皇室, imperial house), there are agnatic cadet branches which split during the course of centuries who received their own family names in order to distinguish them from the main line. They were considered a part of the imperial family (皇族 Kōzoku), with members carrying the title "Imperial Highness", until the laws changed in 1947. The most important branches were the Shinnōke of which the most senior branch Fushimi-no-miya is first in the order of succession. Out of the Fushimi branch the Ōke branches split, which are the Kuni, Kaya, Asaka, Higashikuni and Takeda families as of 2024. Furthermore there are branches created from sons of the emperor who were excluded from the line of succession and demoted into the ranks of the court (kuge) or sword (buke) nobility. Such families are the Minamoto (源 also known as Genji), Taira (平 also known as Heishi), as well as through in-laws the Tachibana for example. Out of these families further branches split through male descent who were also considered noble Japanese clans. The line of legitimate direct male descendants of emperors is therefore numerous.

Other terms used for the dynasty are also Kōka (皇家, Imperial House). Formerly the term Kyūshitsu (宮室, Palace Household) was also used under the old Imperial Constitution and the Imperial Household Law, as well as Teishitsu (帝室, Imperial Household).

The emperor (天皇, tennō) is the head of the Japanese imperial family.

Article 3 and 4 of the Emperor Abdication Law define the Emperor Emeritus and Empress Emerita.

Article 5 of the Imperial Household Law defines the Imperial Family members as the Empress; the grand empress dowager; the empress dowager; the Emperor's legitimate sons and legitimate grandsons in the legitimate male line, and their consorts; the Emperor's unmarried legitimate daughters and unmarried legitimate granddaughters in the legitimate male line; the Emperor's other legitimate male descendants in the third and later generations in the legitimate male line and their consorts; and the Emperor's other unmarried legitimate female descendants in the third and later generations in the legitimate male line.