Saitō Magoshirō was an individual during the Sengoku period.  He was the second son of Saitō Dōsan.  Magoshirō was also known as Tatsushige.  His mother was either Miyoshino or Omi-no-kata.

Magoshirō resided together with his father, Dōsan, his older brother, Saitō Yoshitatsu, and younger brother, Saitō Kiheiji, in Inabayama Castle in Mino Province.

After his coming-of-age ceremony, he assumed the name of Saitō Magoshirō Tatsushige.  Dōsan gave Magoshirō the name of Sakyō-no-suke, and in lieu of Yoshitatsu planned for Magoshirō to become the successor to the family.

Along with his younger brother, Kiheiji, Magoshirō was taken well care of by Dōsan.  This bred arrogance among Magoshirō and Kiheiji, who gradually came to despise their older brother, Yoshitatsu.  In 1554, Yoshitatsu succeeded Dōsan as head of the Saitō family but in anticipation of being removed from the line of succession, Yoshitatsu had devised a plan.  On 11/12 of 1555, Yoshitatsu feigned illness and called Magoshirō, Kiheiji and others to an inner room at Inabayama Castle.  After Magoshirō and Kiheiji became intoxicated with drink, they were murdered by a close retainer of Yoshitatsu named Hineno Hironari (Bitchū-no-kami).

Under an alternate theory, only Magoshirō was killed by Yoshitatsu while Kiheiji survived and became known as the younger brother, Toshitaka.  This theory is subject to question and the two brothers are deemed to have been killed at the same time.