Ii Naomori served as a bushō during the Sengoku period.

Naomori was born as the son of Ii Naomune, the lord of Iinoya Castle in the Inasa District of Tōtōmi Province.  He was a grandson of Ii Naohira.  Naomori’s daughter, Ii Naotora, served as a landlord of Iinoya.  According to the genealogical records of the Ii family, the daughter of Ii Naohira bore Tsukiyama-dono, the wife of Matsudaira Motoyasu (later known as Tokugawa Ieyasu).  As such, Tsukiyama-dono was a younger cousin of Naomori.

The Ii clan were kokujin, or provincial landowners, in Tōtōmi.  In 1494, after Imagawa Ujichika, the military governor of Suruga Province, advanced into Tōtōmi, the Ii combined with the Ōkōchi clan and the Shiba clan (military governors of Tōtōmi) to resist.  This conflict persisted during the Meiō, the Bunki, and the Eishō eras.  In 1513, after the Imagawa garnered control of Tōtōmi, Ii Naohira, a member of the Ii clan, served under the command of the Imagawa and acquired influence in the area of Iinoya.

There are two theories regarding the age of Naomori at the time of his death, either thirty-five or fifty-five.  Given that his grandfather, Naohira, was either seventy-five or eighty-five at the time of his death in 1563, it is not likely that his grandson was fifty-five years old in 1560, so it is surmised that he was thirty-five years old and was born in 1526.  If Naomori’s daughter, Naotora, is presumed to have been born around 1536, then, if Naomori was born in 1526, he would have been only around eleven years old at the time of her birth which is too young so his date of birth is unknown.

Naomori served Imagawa Yoshimoto, a sengoku daimyō and the eleventh head of the Imagawa clan.  In the era of Yoshimoto’s father, Imagawa Ujichika, the Imagawa established control over Suruga and Tōtōmi provinces.  Yoshimoto added Mikawa Province, ushering in the period of peak prosperity of the clan.  In 1560, Yoshimoto mobilized a large army for an expedition to Owari Province.  At this time, Naomori was in charge of the vanguard forces.

Initially, the Imagawa army seized the initiative by capturing multiple bases of the Oda, but on 5/19, while the Imagawa army rested in Okehazama in Oda-controlled territory in the Chita District of Owari, Oda Nobunaga launched a surprise attack with a small force that overwhelmed the Imagawa, resulting in many losses including the death of their lord, Yoshimoto.  This is known as the Battle of Okehazama.

In this battle, Naomori, along with sixteen retainers, was killed in action.  Naomori was buried at the Ryūtai Temple, the family temple of the Ii.  Based on his posthumous name, the temple was renamed as the Ryōtan Temple.

Naomori died without a natural heir so was succeeded by his adopted younger cousin, Ii Naochika.  At the time, kunishū across Tōtōmi rebelled against the Imagawa clan in an event known as the Enshū Discord.

Upon the direction of Ii Naohira, the prior head of the Ii clan, Nakano Naoyoshi was assigned as the guardian of Naochika in accordance with the terms of Naomori’s will.  He then used the name of Shinano-no-kami which had previously been used by Naohira and Naomori.  Naochika had poor relations with Ono Michiyoshi, a retainer of the Ii.  Michiyoshi plotted to seize Iinoya but because Naoyoshi served as guardian for Naochika, he could not achieve his goal.  Having learned of Naochika’s collusion with the Tokugawa and Oda, in 1562, Michiyoshi brought a claim to Imagawa Ujizane of Sunpu, alleging that Naochika was colluding with the enemy and planning to rebel against the Imagawa.

After Ujizane attempted to raise an army to pursue Naochika, Naochika headed toward Sunpu to apologize to Ujizane but, on the way along the Kake River, he was murdered by a retainer of the Imagawa named Asahina Yasutomo.  Upon orders from Ujizane, Michiyoshi tried to kill Naochika’s orphan, Toramatsu (later known as Ii Naomasa), but was blocked by Niino Chikanori, a senior retainer of the Imagawa close to the Ii family.  Thereafter, in 1563, Ii Naohira died and, in 1564, Naoyoshi was killed in battle.  Prior to his demise, Naoyoshi had been overseeing Iinoya from Iinoya Castle, so Jirō-Hōshi (Ii Naotora), Naomori’s daughter, became the next lord of the castle.  After Matsudaira Motoyasu abandoned the Imagawa, allied with the Oda, and invaded Tōtōmi, Naotora fell under the command of the Matsudaira.  Later, Naochika’s orphan, Ii Naomasa, became a senior retainer of Ieyasu and was counted among the Four Divine Kings of the Tokugawa.