Niki Mitsunaga (仁木 満長) was a samurai lord of the late Nanboku-chō and early Muromachi period. He was the Governor of Ise Province (1390-1396).

Niki Mitsunaga was born as the son of Governor of Ise Province Niki Yoshinaga of the Nitsuki clan (also known as Niki clan).^[1][2] After Yoshinaga's elder brother Niki Yoriaki became a steward of the Ashikaga shogunate, aided by his political power, the Nitsukis simultaneously served as governors of nine provinces in the early Nanboku-chō period. However, after 1372, when Tajima Province was taken over by the Yamana clan, the Nitsukis lost all their provinces.^[2]

In 1390, Mitsunaga was appointed as Governor of Ise Province after the Toki clan was deposed by Ashikaga Yoshimitsu due to the Mino Rebellion.^[1][2]

However, in July 1396, his illegitimate elder brother Niki Yoshikazu took the position over, forcing Mitsunaga to leave the political scene. Shortly after this, he was reported to have become a fugitive and gone missing in Ise Province and his fate remains unknown. This shift in power was a conspiracy plotted by Yoshikazu's close retainer, Yūki Mitsufuji. Enraged by this, the Shugo Daimyo approached Yoshikazu and had Mitsufuji exiled.^[2]

The Nitsuki clan descended from Emperor Seiwa (850-881) through the Seiwa Genji, the most successful and powerful line of the Minamoto clan, via the Ashikaga clan.^[3]

Although Mitsunaga's descendants did not become Governors, they survived as what became known as the Ise Nitsuki family.^[2]