Title: Sagami no Kami
Also known as: Matsuchiyomaru (松千代丸—childhood), Shinkurou (新九郎—nickname), 慈雲院松巌傑公 (posthumous)

Ujimasa was born in 1538 as the second son of Houjou Ujiyasu and his principle wife Zuikeiin, daughter of Imagawa Ujichika, and was older brother of Houjou Ujiteru, Houjou Ujikuni, Houjou Ujinori, Houjou Ujitada, Houjou Saburou (Uesugi Kagetora), and Houjou Ujimitsu. He became heir to the clan when his older brother Shinkurou died before reaching adulthood.

Ujimasa married Oubaiin, eldest daughter of Takeda Shingen and Sanjou-no-Kata, on the occasion of the three-way alliance between the Takeda, Imagawa, and Houjou clans in 1554. Their marriage was thought to be a happy one.

Ujimasa succeeded his father as the fourth head of the Sagami Houjou Clan in 1559 upon Ujiyasu's retirement. His first task upon becoming heir of the clan, per clan convention, was a a land survey evaluating how the Houjou lands were being used and the condition of the people serving on those lands. His relationship with his brothers was good throughout, and they were be a huge help to him in the governing of the clan.

In 1561, Uesugi Masatora (Uesugi Kenshin) of Echigo laid siege to Odawara Castle with a huge army gathered from the Kantou and south Mutsu. Under the leadership of his father Ujiyasu, Ujimasa was able to drive back the army. After the Fourth Battle of Kawanakajima, he was able to take back a large part of North Kantou from the Uesugi in concert with Shingen.

In 1568, seizing the opportunity presented by the decline of the Imagawa Clan after Imagawa Yoshimoto's death at Oda Nobunaga's hand, Takeda Shingen invaded Suruga, laying siege to Yoshimoto's heir, Imagawa Ujizane in Kakegawa Castle. Ujimasa led the Houjou forces to repel the Takeda army and formed an alliance with Tokugawa Ieyasu of Mikawa in order to rescue Ujizane (his brother-in-law by way of his younger sister Hayakawadono). Ujimasa then had Ujizane adopt his son Ujinao as his heir, thus giving the Houjou Clan a legitimate claim to the territory of Suruga. In order to hold back Takeda, he formed an alliance with his old enemy Uesugi Kenshin, giving his younger brother Saburou (Uesugi Kagetora) as hostage. The severing of ties with the Takeda Clan, however, meant the dissolution of his marriage with his beloved wife Oubaiin.

In 1569, Takeda Shingen laid siege to Odawara Castle, delivering a crushing defeat to the Houjou Clan (though recent analysis by historians indicate that Shingen lost a great many men as well). In 1570, Suruga belonged almost wholly to Shingen.

In October of 1571 upon his father's death, Ujimasa broke off his alliance with Kenshin and reformed the alliance with Shingen in accordance with his father's will, after which fighting between the Houjou and Uesugi clans flared up again.

Kenshin's death in 1578 triggered a fight for succession to the Uesugi Clan between his two adopted sons, Uesugi Kagekatsu and Uesugi Kagetora (the Otate no Ran). Ujimasa was tied up at that time in a confrontation with Satake Yoshishige and Utsunomiya Kunitsuna in Shimotsuke, so sent his brother Houjou Ujikuni to their brother's aid in his place while asking Takeda Katsuyori for reinforcements. Katsuyori betrayed the Houjou and formed an alliance with Uesugi Kagekatsu, and the Otate no Ran ended with Kagetora's death and Kagekatsu's succession.

Ujimasa broke off the alliance with the Takeda clan a second time and formed an alliance with Oda Nobunaga and Tokugawa Ieyasu to attack the Takeda territory in a pincer movement, but shifting alliances and hard fighting left the conclusion unclear. In 1580 Ujimasa proposed to Oda Nobunaga, who had just taken Ishiyama Hongan Temple, that the Houjou Clan become a vassal of the Oda Clan, but Takeda Katsuyori managed to form an alliance with Oda first. Ujimasa retired from the position of clan head in the same year, but like his father before him still held onto the government and military affairs of the clan.

In the following years, the Houjou Clan managed to gain control over a vast territory: Sagami, Izu, Musashi, Shimousa, Kazusa, Hitara, Shimotsuke, and a part of Suruga. Interestingly, however, Ujimasa did not seem to hold the ambition of ruling the entire country, a tradition passed down from the founder of the Late Houjou Clan, Houjou Souun. Instead, Ujimasa concentrated on independence for the 8 Kantou provinces under Houjou rule and alliances with other strong warlords such as Tokugawa Ieyasu and Date Masamune.

In 1589, using Ujimasa's refusal to proceed to the capital to attend him as pretext, Toyotomi Hideyoshi gathered an army of 220,000 to lay siege to Odawara Castle. It overran castles in the Houjou territory in quick succession. The siege against Odawara Castle lasted from May to August. On August 4, Ujimasa offered to surrender his life for the lives of his men. Toyotomi demanded the lives of both Ujimasa and his brother Ujiteru, as well as the lives of their vassals Matsuda Norihide and Daidouji Masashige. Ujimasa and Ujiteru committed seppuku on August 10.

Ujimasa left behind the following tanka verses for his death poem:

「雨雲の　おほえる月も　胸の霧も　はらいにけりな　秋の夕風」
「我身今　消ゆとやいかに　おもふへき　空よりきたり　空に帰れば」

translated (Sadler 1978, pp. 160–161):

Autumn wind of eve,
blow away the clouds that mass
over the moon's pure light
and the mists that cloud our mind,
do thou sweep away as well.

Now we disappear,
well, what must we think of it?
From the sky we came.
Now we may go back again.
That's at least one point of view.

There is another verse which is sometimes attributed to his brother Ujiteru, but is most often attributed to Ujimasa:

「吹くと吹く　風な恨みそ　花の春　もみじの残る　秋あればこそ」

which may be translated:

The wind's resentment—
Oh, see how it blows against
The flowering spring.
Yet it will leave us anon
The bright colors of autumn.