Rereahu was a Māori rangatira (chieftain) of Ngāti Raukawa in the Tainui tribal confederation from the Waikato region, New Zealand. He probably lived in the first half of the seventeenth century. He is the ancestor of the Ngāti Maniapoto, Ngāti Hauā, and Ngāti Korokī Kahukura iwi, and of Rereahu, a group based around Maniaiti / Benneydale, Pureora, and Maraeroa in Waitomo District, whose status as a separate iwi or as a hapū (sub-tribe) of Ngāti Maniapoto is a matter of dispute.

Rereahu’s father was Raukawa, the son of Tūrongo and Māhina-a-rangi, and a direct male-line descendant of Hoturoa, leader of the Tainui waka. His mother was Turongoihi.^{[citation needed]} He had three younger brothers: Kurawari (father of Whāita and Korokore), Whakatere, and Takihiku (father of Tama-te-hura, Upoko-iti, Wairangi, and Pipito).

There was a tribe called Ngāti Hā, led by three chiefs, Hā-nui ('Big Hā'), Hā-roa ('Long Hā'), and Hā-kūhā-nui ('Big-thigh Hā'), who had been driven out of the Taupō region by Ngāti Tūwharetoa and headed west, establishing a village on the Mōkau River, upstream from Puketutu. Rereahu noticed the Ngāti Hā at Te Tīroa while he was foraging for mamaku shoots and reported to his third-cousin Tamāio that they were coming to seize the land. As a result, Tamāio raised a war party, which advanced on the Ngāti Hā village and drove them out of the region.

Rereahu married Rangi-ānewa, daughter of Tamāio. They settled in the village called Tihikoreoreo, next to Waimiha,^{[citation needed]} where they had one son:

Rereahu later married Hine-au-pounamu, whose parents were Tū-a-tangiroa and a daughter of the Ngāti-Hā chief Hā-kūhā-nui. Tū-a-tangiroa was a son of Uenuku-tuhatu Uetapu, the older brother of Tamāio’s father Uenuku-te-rangi-hōkā, which meant that Hine-au-pounamu was senior to Rangi-ānewa, which had implications for the relative status of Rereahu’s children. From this marriage, there were six sons and two daughters:

These children were raised in the region around Kāwhia. Subsequently, they settled along the Waipā River and the Manga-o-kewa Stream, with a central hub at Te Kūiti. Rereahu is depicted on the front post of Te Tokanganui-a-noho marae at Te Kūiti. Rereahu himself settled at Ngā Herenga in Maraeroa, where he lived until his death. The location remains a wāhi tapu (sacred space) for his descendants.

When Rereahu was on his death-bed he decided to give his mana to Maniapoto, rather than Te Ihinga-a-rangi, because he thought the younger brother had proven himself a better leader. Therefore, he told Te Ihinga-a-rangi to go to the tuahu (altar) and perform the rituals, promising to pass the mana to him when he returned. While he was away, he called Maniapoto to him, covered his head in red ochre and instructed him to bite the crown of his head, passing the chiefly mana to him. Maniapoto objected, but Rereahu declared that Te Ihinga-a-rangi was illegitimate in some way. Pei Te Hurinui Jones suggests that this was because Rereahu already planned to marry Hine-pounamu when Te Ihinga-a-rangi was conceived and/or because Hine-moana was genealogically senior to Rangi-ānewa. Maniapoto accepted the mana and by the time Te Ihinga-a-rangi returned, Rereahu was dead. This led to a conflict between the brothers, in which Maniapoto was victorious.

The Rereahu tribal group are descended from Rereahu. Their rohe centres on Mangapeehi Marae / Rereahu Wharenui near Maniaiti / Benneydale, and Te Hape Marae / Te Kaha Tuatini Wharenui near Pureora. They are also among the hapū that share Otewa Pā Marae / Aroha Nui Wharenui near Ōtorohanga, and of Te Ahoroa Marae / Tapairu Wharenui near Te Kūiti. The Ngāti Raukawa branch of Rereahu is based at Ōwairaka Rāwhitiroa Marae / Takihiku Wharenui near Parawera.