Munisai Hirata, also called Shinmen Munisai, gained renown as a martial artist mastering kenjutsu and juttejutsu. He served the Shinmen clan, led by Lord Shinmen Iga no Kami of Takayama Castle in Mimasaka Province. His victory over Yoshioka Kenpō, founder of the Yoshioka-ryū, in a Kyoto duel earned him the Shōgun Ashikaga Yoshiteru's title "Unrivaled Under the Sun." This triumph later sparked a feud between his son, Miyamoto Musashi, and the Yoshioka family.

His lineage descended from Hirata Shōgen, a Shinmen clan vassal, granting Munisai the privilege to use the Shinmen name. He founded the Tōri Jitte Ryū school, focusing on sword and jutte combat. Historical records conflict regarding Musashi's parentage: some indicate Munisai was Musashi's biological father, while others suggest Musashi was adopted from the Akamatsu clan after Munisai's first wife, Omasa, died in childbirth. Alternative accounts propose Musashi was born to Munisai's first wife, Yoshiko, before their divorce, after which Yoshiko left Musashi with Munisai. Munisai later married Omasa, who raised the boy.

Munisai trained Musashi in swordsmanship and jutte from childhood, though their relationship was reportedly strained. In 1589, Shinmen Sokan, head of the Shinmen clan, ordered Munisai to execute his own student, Honiden Gekinosuke. After fulfilling this command, local backlash forced Munisai to relocate to Kawakami village, interrupting Musashi's training under him.

By the late 1590s, Munisai entered the service of the Kuroda clan under Kuroda Toshitaka. He fought for the Tokugawa forces at the 1600 Battle of Ishigakibara. Later, he retired, became a Buddhist monk adopting the name Munisai, and moved to Kitsuki in Bungo Province. There, he refined his Tōri-ryū and interacted with local lords, including Kinoshita Nobutoshi, as noted in Nobutoshi's 1613 diary. Munisai likely died in the 1620s, though his gravesite remains unknown.