Niiro Tadamasu served as a bushō during the Azuchi-Momoyama and early Edo periods.  He was a retainer of the Shimazu clan of Satsuma Province.

Tadamasu was born as the second son of Niiro Tadamoto.  His older brother, Niiro Tadaaki, was killed in action in 1583 so Tadamasu became the lineal heir to his father.

In 1584, at the Battle of Okitanawate, Tadamasu tried using a long sword for the first time while fighting on the front lines.  In 1586, serving as a proxy for his father, Tadamasu invaded Bungo Province.  This event is known as the Hōsatsu War.  Together with Hirata Sōgi, they expanded their territory.  Tadamasu also aided the pacification of a rebellion in Higo.  After the Shimazu clan surrendered to Toyotomi Hideyoshi, Tadamasu was sent as a hostage to Kyōto.  Later, he served in the Bunroku-Keichō Campaign on the Korean Peninsula.

In 1600, when Ishida Mitsunari launched a revolt, Tadamasu joined Shimazu Yoshihiro to serve on behalf of the Western Army.  Owing to a defeat at the Battle of Traverse Kōda-Kisogawa on 8/22, Mitsunari decided to withdraw to Ōgaki Castle, but a portion of the Shimazu forces including Shimazu Toyohisa were encamped at Sunomata so there was a risk they would become isolated.  Shimazu Yoshihiro asserted that they should withdraw after having Toyohisa and the others withdraw first, but Mitsunari would not listen to him.  Tadamasu, together with Kawakami Hisatomo, took away the bit from Mitsunari’s horse and criticized his actions, but Mitsunari still did not accept their opinion.

Tadamasu participated in the clashes during the withdrawal after the Battle of Sekigahara.  Later, he was appointed to serve as the lord of the manor of Yamada in Ōsumi Province.

In 1608, Tadamasu died in Yamada.  Tadamasu’s lineal heir, Niiro Tadakiyo, earlier became the adopted son-in-law of Tadamasu’s older brother, Tadaaki, so Tadamasu’s second son, Niiro Tadamitsu, inherited the headship of the clan.