Niida Masamori served as a bushō during the Sengoku and Azuchi-Momoyama periods.  He was a retainer of the Nanbu clan of Mutsu Province.

In 1547, Masamori was born as the son of Niida Yukimasa.

After his father, Yukimasa, joined with Nanbu Harumasa to overthrow the Hachinohe (based at Ne Castle) who were the main branch of the clan, Masamori’s older brother, Hachinohe Masayoshi, became the head of the Hachinohe clan.  As a result, Masamori inherited the headship of the Niida family.  Thereafter, Masamori and Masahide backed Nanbu Nobunao as the successor to the Nanbu clan and participated on the side of the forces to suppress the Revolt of Kunohe Masazane.  He served as a senior retainer of the Hachinohe.

Masamori died in 1603.