- Birth: 1494
- Death: 1556
- Other names: Matsunami Shôkurô, Nishimura Kankurô, Nagai Yorihide, Saitô Hidetatsu, Saitô Toshimasa
- Titles: Sakyô-daibu, Jibu-Ôsuke
- Distinction: Lord of Mino
- Sons: Saitô Yoshitatsu, Magoshiro, Saitô Nagatatsu
Saitô Dôsan's origins are unclear. According to the traditional story of his life, he was born in Yamashiro province and was the illegitimate son of Matsuda Motomune. After attempting a career as an oil-seller, he assumed the name Nishimura Kankurô and entered the service of Nagai Nagahiro of Mino province. According to another version, Toshimasa's father, a certain Shinzaemonjô, a monk at the Myoukaku-ji in Kyoto, had given up the priesthood and married the daughter of an oil merchant. He picked up the trade and in the course of hawking his wares on the road happened to run into a friend from his days as a monk at the temple. This acquaintance was now the abbot of the Jouzai-ji in Mino province. The abbot was a relative of the Nagai clan, retainers of the Toki, the shugo of Mino Province who were at that time led by Toki Masafusa (1467-1519). With the abbot's recommendation, Shinzaemonjô gained employment with Nagai Nagahiro, and quickly rose through the Nagai ranks, eventually taking the surname of Nagai himself.
Regardless of his roots, Dôsan contributed to a general instability within Mino Province and around 1526 Toki Yoshinari gave him his concubine in the hopes that this would appease him. He officially succeeded his father in 1533 and overthrew Nagai in 1542 and took control of the Mt. Kinka area. An ambitious schemer, he next overthrew the Toki in 1544 and established himself as daimyô, building Inokuchi (Inabayama) Castle. Many of the details of Toshimasa's career to this point are obscure but he certainly was a ruthless schemer who murdered a number of his superiors in his rise. He fought with Oda Nobuhide of Owari province and dealt him a defeat at Kanoguchi in 1547. Peace was made the following year when it was agreed that Toshimasa's daughter would marry Nobuhide's son Nobunaga. Toshimasa's eldest son was Yoshitatsu. Yoshitatsu's mother was the concubine of Toki Yoshinari and as he was born less than nine months after the women was given to Toshimasa, his parentage was unclear. Despite establishing Yoshitatsu in Inabayama while he settled in Sagiyama, Dôsan planned to name one of his other sons heir. In response Yoshitatsu killed two of his brothers and went to war with his Toshimasa in 1556. Toshimasa and Yoshitatsu met at the Nagara River (Battle of Nagaragawa) on the 20th day of the 4th month of 1556 and in the course of the fighting, in which Toshimasa was heavily outnumbered, Toshimasa's head was taken by a certain Komaki Genta. Toshimasa is alleged, as a result of his desperate circumstances, to have named Nobunaga as lord of Mino Province in his will and sent this document to Nobunaga. Nobunaga, however, was unable to provide his father-in-law with aid. A colorful character, Dôsan also had a reputation for cruelty and was considered by many to be a highly unsavory figure, earning the nickname mamushi (viper). His wife, known as Ômi no kata, was a daughter of Akechi Suruga no kami Mitsutsugu. Toshimasa had adopted the name Saitô, former shugodai of Mino overcome by the Nagai in the 1520s.
- Initial text from Sengoku Biographical Dictionary (Samurai-Archives.com) FWSeal & CEWest, 2005
- Kanaya, Shunichiro. Sengoku Jidai ga Omoshiroku Wakaru Hon (戦国時代が面白くわかる本) Japan, 2003