Hōjō Ujiyasu (北条 氏康; 1515 - October 21, 1571) was the son of Hōjō Ujitsuna and daimyō of the Hōjō clan. His wife was Suikeiin, sister of Imagawa Yoshimoto.
After his father's death in 1541, numerous enemies of the Hōjō took the opportunity to assault major Hōjō strongholds. Two Uesugi factions united with the Koga kubō attacked Kawagoe in 1542 in a nighttime clash still celebrated by Japanese military annals.
"The result was a complete defeat of the Uesugi and the Koga contigent. From that day on, the Hōjō clan began a series of triumphs, beginning with the destruction of the Uesugi family."
Ōgigayatsu Tomosada unsuccessfully attempted to conquer Edo Castle, and a few years later, in 1545, an army led by Ashikaga Haruuji (足利 晴氏) and Uesugi Norimasa besieged Kawagoe Castle.
Hōjō Tsunashige (北条 綱成), stepson of Ujiyasu's brother Tamemasa (北条 為昌), and Ujitsuna's son-in-law, were outnumbered by 3,000 against 80,000 while Ujiyasu led a reinforcement force of 8,000 soldiers.
Ujiyasu managed to get a samurai to pass through the enemy lines to inform Tsunashige of the enemies' approach, and made use of ninja to learn the enemy's strategies.
With great daring he led a night attack on the Ashikaga-Uesugi forces. Despite being greatly outnumbered, the Hōjō army defeated the besiegers because on Ujiyasu's orders no heavy armor was worn and no one stopped to collect trophy heads.
This battle led to the end of the Ōgigayatsu branch of the Uesugi clan and destroyed the prestige of Norimasa of the Yamanouchi-Uesugi branch as Kantō Kanrei (vice-shogun of the Kantō) of the region, at least until the arrival of Uesugi Kenshin.
Allied with Takeda Shingen in 1563, they conquered Matsuyama Castle in Musashi Province.
Hōjō Ujiyasu expanded the Hōjō territories, which now covered five provinces, by managing and maintaining what his father and grandfather had conquered. He conquered Kōnodai in 1564 after a battle against Satomi Yoshihiro.
Towards the end of his years a larger conflict began between his clan and Takeda Shingen, who wanted to become the greatest daimyō of that time.
In response to the Hōjō invasion of Suruga Province, Shingen entered Musashi Province from the Kai, attacking Hachigata and Takiyama Castle, where Ujiyasu's sons pushed him back.
However, despite having several intact enemy fortresses behind his army, Shingen pushed on to the Hōjō stronghold of Odawara, burning the town around the castle and retreating after three days.
Two of Ujiyasu's seven sons clashed with the Takeda at the Battle of Mimasetōge in 1569, thus ending the first Takeda campaign against the Hōjō.
To end the turbulent period, Ujiyasu sought to build a peace with Uesugi Kenshin and Takeda Shingen, the Hōjō's strongest opponents, by letting Kenshin adopt his seventh son and accepting the fait accompli of Shingen's rule over Suruga province.
To weld the alliance between the Takeda, Imagawa and Hōjō, Ujiyasu gave his two daughters to the two clans; lady Hayakawa became the wife of Imagawa Ujizane while lady Hōjō (Hōjō Masako) became the second wife of Takeda Katsuyori.
Ujiyasu died in 1571, passing the leadership of the clan to his eldest son Ujimasa in a relatively favorable situation.