Zejo was the title of the chieftain of the Tachibana and O clans in the Heian period.
Zejo was the title of a head of clans that branched out of the Imperial family, especially the Tachibana clan. In the Heian period, the chieftains of the Imperial family, the O clan, as well of the Minamoto, Fujiwara and Tachibana clans, were authorized to recommend grant of noble ranks to members of their claims in the uji no shaku ceremony. However, the decline of the Tachibana clan led Sangi (Councilor) TACHIBANA no Tsunehira to become the last kugyo (court noble) in his clan and the authorization was delegated to Dainagon (Major Counselor) FUJIWARA no Michitaka, the son of FUJIWARA no Tokihime (the lawful wife of FUJIWARA no Kaneie) who was the daughter of Dainagon TACHIBANA no Sumikiyo. The person holding the authorization was called the Zejo. Since then, the Fujiwara and Minamoto clans who were maternal relatives of the Tachibana clan were granted this title under imperial decree given to uji no in, based on recommendation by the Tachibana clan.
This title was used also within the imperial O clan and granted to the first Imperial Prince and later to choja (chieftain of the family) selected from the Shoo (princes without proclamation as imperial princes); however it moved to the Jingihaku (administrators of religious affairs) with the drop in the number of Shoo after the Heian Period, inheriting the post over generations except for temporary restoration of the title to the Imperial Princes.
Hosho (document announcing the decision of ruling class to lower-ranking people) written by the zejo was called zejo-sen.