Murai Seizō served as a bushō during the Azuchi-Momoyama period.
Seizō was a family member and retainer of Murai Sadakatsu. During the Oda administration, Sadakatsu served as the Kyōto shoshidai, or official in charge of the Board of Retainers, an office in the Muromachi bakufu to guard the shōgun and give judgment on criminals in Kyōto.
The first appearance of Seizō in historical records is in regard to his participation in a banquet at the Imperial palace on 8/3 of Genki 1 (1570). On this occasion, Seizō played the taiko, or large drum, in a musical performance.
After Sadakatsu became the Kyōto shoshidai and established a residence in Kyōto, Seizō followed him by moving to the capital where he supported Sadakatsu including with the reception of visitors. He was also dispatched by Sadakatsu to meet Yoshida Kanemi. Although the details are uncertain, in the winter of 1578, he angered Sadakatsu’s eldest son, Murai Sadanari, and was ousted. He then lived in seclusion at the Mii Temple. On 1/2 of Tenshō 7 (1579), Seizō received a visit by a Shintō priest sent by Kanemi to console him.
Nevertheless, in Tenshō 7 (1579), it appears that Seizō was pardoned and, on 6/9 of the same year, he received medicines and incense from Yamashina Tokitsune to assist Sadakatsu during convalescence at home. On the same day, he received a visit from Yoshida Kanemi to discuss land rents for hunting. Sadakatsu and Kanemi promised to attend a performance of sarugaku, or circus theater, at the Konren Temple on 7/8, but Sadakatsu could not attend so Seizō was sent on his behalf. At the end of the twelfth month, Seizō implemented the verdict of Sadakatsu with respect to a claim made by Kanemi against Hirano Kaneoki in regard to land rent for hunting.
On 3/12 of Tenshō 8 (1580), Seizō guided a messenger of the Gohōjō clan to the Nijō palace, serving as a close associate of Sadakatsu.
On 6/2 of Tenshō 10 (1582), perhaps because Seizō was not accompanying Sadakatsu, he survived the coup d’état against Oda Nobunaga known as the Honnō Temple Incident.
After the Battle of Yamazaki, on 6/22, upon orders of Oda Nobutaka, Seizō and Kuwabara Sadanari oversaw the construction of burial mounds for the heads of Akechi Mitsuhide and Saitō Toshimitsu in Awataguchi in Kyōto. On 6/23, Seizō provided notice of an order to examine and submit the possessions of Mitsuhide held by his followers and those of Konoe Sakihisa. Sakihisa, a high-ranking noble, was dispirited by the unexpected demise of Nobunaga, undergoing the rites of tonsure and adopting the monk’s name of Tatsuyama. He was, however, the subject of false charges that, on the day of the coup, soldiers in Mitsuhide’s army fired shots at the Honnō Temple from Sakihisa’s residence. This resulted in questioning from Oda Nobutaka and, later, Hashiba Hideyoshi.