Akamatsu Masanori served as a bushō during the Sengoku and Azuchi-Momoyama periods.  He was a member of the Shichijō-Akamatsu clan and served as the lord of Kōzuki Castle in the Sayō District of Harima Province.

Masanori was born as the second son of Akamatsu Masamoto.  Similar to his father, he was called Nishi-Harima-dono and excelled in the military arts.  He was a lord of prudent judgment with a calm demeanor.  While relationships with the Mōri and Oda families evolved over a period of years, Masanori was considerate and carried a sense of responsibility toward retainers who demonstrated foresight.  This deference also caused him to be viewed as indecisive at times.

In 1577, when Hashiba Hideyoshi assaulted Kōzuki Castle with an army of approximately 30,000 soldiers, he intercepted them with an army of 10,000 men comprised of 7,000 from the main division of the Akamatsu army and 3,000 reinforcements from Ukita Hiroyuki, the lord of Kameyama Castle and underling of Ukita Naoie.  Nevertheless, after a ferocious attack by the Oda army, a defeat appeared imminent.  First, Masanori stabbed his wife, then he and other followers of the family took their own lives.  Among those officers and soldiers martyred were Takashima Masazumi, Hayase Masayoshi, Ukita Hiroyuki, Kōdera Nyūdō, and Nakamura Ise Nyūdō.

According to one theory, he told a retainer who was assigned to decapitate him “Take my head to the Oda and surrender.  If Chikuzen-dono (Hideyoshi) does not accept it, then lure them in to kill on the spot.”  The defeated soldiers took the head to Hideyoshi and Kuroda Yoshitaka and pleaded for their lives but Hideyoshi rejected their pleas and, upon orders of Yoshitaka, all of those from the castle were slaughtered.