Ii Naomitsu served as a bushō during the Sengoku period.  He was a retainer of the Imagawa clan.

Naomitsu was born as the second (or third) son of Ii Naohira, the lord of Iinoya Castle in the Inasa District of Tōtōmi Province.

The Ii clan were kokujin, or provincial landowners, in Tōtōmi.  Naomitsu, together with his father, Naohira, served Imagawa Yoshimoto.  Ii Naomori (Naomitsu’s nephew and the lineal heir of his older brother, Ii Naomune) did not have a son so Naomitsu promised to send his own son, Kamenosuke (later known as Ii Naochika), as an adopted heir of Naomori.  This, however, invited opposition from retainers who were against the succession by Naochika to the headship of the clan.  In particular, Naomitsu did not for some time get along well with Ono Masanao, a chief retainer of the Ii family.  After the Kai-Takeda army began to encroach upon the territory of the Ii family, acting upon orders of their father, Naohira, Naomitsu and his younger brother, Ii Naoyoshi, began military preparations.  Masanao then slandered them by telling Imagawa Yoshimoto that Naomitsu and Naoyoshi were traitorous.  On 12/23 of Tenbun 13 (1544), Naomitsu and Naoyoshi were summoned to Sunpu to explain themselves to Yoshimoto but were murdered instead.