Seshū Shinnōke (世襲親王家) was the collective name for the four cadet branches of the Imperial Household of Japan, which were until 1947 entitled to provide a successor to the Chrysanthemum throne if the main line failed to produce an heir. The heads of these royal houses held the title of imperial prince (親王 shinnō), regardless of their genealogical distance from the reigning Emperor, as the term seshū in their designation meant that they were eligible for succession.
The Imperial family of Japan considers itself a single dynasty in unbroken succession; however, the succession has often not been directly from father to son, but has been in the male line within a closely related group of people. In the Muromachi period, Prince Yoshihito, the son of the Northern Emperor Sukō was permitted to establish a parallel lineage to the main imperial line, and took the name Fushimi-no-miya from the location of his palace. This served politically to cement the reunification of the Northern and Southern Court, but provided insurance in the extreme event that the main imperial line should fail to produce a direct heir and become extinct.
This proved to be a fortunate decision, as in 1428, the son of the 2nd Prince Fushimi-no-miya ascended the throne as Emperor Go-Hanazono.
In the Edo period, three additional seshū shinnōke households were created by the Tokugawa shogunate, in conscious imitation of the Tokugawa Gosanke (徳川御三家)
However, aside from Emperor Go-Hanazono, the only time a member of the seshū shinnōke ascended to the throne was in 1779, when the son of Prince Kan'in-no-miya Sukehito became Emperor Kōkaku.
Within the seshū shinnōke households, younger non-heir sons (who were titled prince (親王 shinnō)), had two career options. They could "descend" to subject status with a surname such as Minamoto or Taira, and serve as a government official, or they could enter the Buddhist priesthood, generally as the head of one of the monzeki temples in and around Kyoto. During the Edo period, the latter practice became almost universal. Non-heir sons who entered the priesthood were styled princely priest (法親王 hoshinnō), and were automatically excluded from the succession, but could be recalled to "secular" status (and thus reinstated as potential successors) as need arose. Daughters were frequently used as pawns to cement matrimonial alliances with kuge, daimyō or the Tokugawa houses. Unwed daughters often became Buddhist nuns.
During and after the Meiji Restoration, members of the seshū shinnōke often served in the Imperial Japanese Army or Imperial Japanese Navy.
The four seshū shinnōke were, in order of creation:
The Katsura-no-miya and Arisugawa-no-miya houses died out in 1881 and 1913, respectively. The sixteenth son of Prince Kuniie, the twentieth head of the Fushimi-no-miya, succeeded to the Kan'in-no-miya house in 1875.
The Fushimi-no-miya house was the progenitor of nine other cadet branches of the imperial family, the ōke, during the reign of Emperor Meiji.
The seshū shinnōke and ōke households, along with the kazoku (Japanese peerage), were reduced to commoner status during the American occupation of Japan, in October 1947.