Tsutsui Junsei served as a bushō during the Sengoku period.
Junsei was born as the second son of Tsutsui Junkō, a sengoku daimyō in Yamato Province.
In 1550, Junsei’s older brother, Tsutsui Junshō, suddenly died from illness. Junshō’s son, Tsutsui Junkei, was still in his youth so Junsei served as his proxy and guardian, looking after affairs of governance for the Tsutsui clan. Initially, another family member named Fukuzumi Munemoto is deemed to have served as the guardian for Junkei.
In this era, the Tsutsui and Miyoshi clans wielded authority in the Kinai and were allies. In a letter surmised to have been from 1554, Junkei sent reinforcements to Miyoshi Nagayoshi. In the second month of 1557, Junsei strengthened mutual ties with the Miyoshi, holding a celebration with Hatakeyama Takamasa (the military governor of Kawachi) who, similar to the Tsutsui, backed the Miyoshi.
The Tsutsui family was divided with one faction led by Fukuzumi Munemoto respecting the Jimon school of the Tendai sect and warrior monks at the Kōfuku Temple and the other faction, led by Junsei, loyal to their ties to Kawachi. In the twelfth month, Junkei was expelled from Yamato by the faction led by Munemoto whereupon he entered Iimoriyama Castle, the residence of Yasumi Munefusa, a senior retainer of the Hatakeyama clan.
In the second month of 1558, Junkei, together with Munefusa, entered Nara and worshiped at the Kasuga Grand Shrine. With the backing of Munefusa, Junkei was able to return to Yamato. Thereafter, it appears that Junsei replaced Fukuzumi Munemoto as the guardian of Junkei. In the sixth month of 1559, Munemoto entered the priesthood. Around this time, Junsei became the representative of the monks from the Kōfuku Temple in Nara and de facto head of the Tsutsui clan.
On 8/4 of Eiroku 2 (1559), Miyoshi Nagayoshi attacked Munefusa who earlier ousted Nagayoshi’s lord, Hatakeyama Takamasa. Moreover, Matsunaga Hisahide, a retainer of the Miyoshi, commenced an invasion of Yamato with the aim of eliminating the Tsutsui clan who had ties with Munefusa. On 8/6, Tsutsui Castle fell to Hisahide while Junsei retreated to Tsubaokami Castle in Nara.
Thereafter, the Tsutsui forces led by Junsei battled back and forth against the Matsunaga army on repeated occasions. On 3/19 of Eiroku 7 (1564), in the midst of these clashes, Junsei died in Sakai in Izumi Province.