The Siege of Odawara in 1590 was the campaign by which Toyotomi Hideyoshi killed Houjou Ujimasa, exiled his son Houjou Ujinao, and eliminated the mighty Later Houjou Clan, ruler of the eight provinces of the Kantou, as a threat to his power.

The Houjou were not caught unprepared; guessing at Hideyoshi's intentions, they had made preparations by making a massive recruitment effort targeting men from 15 to 70 years of age, shoring up arms, and making large-scale renovations and repairs to Odawara Castle, Hachiouji Castle, Yamanaka Castle, Nirayama Castle, and others.

Toyotomi's main force consisted of Toyotomi Hidetsugu, Tokugawa Ieyasu, Oda Nobukatsu, Gamou Ujisato, Kuroda Yoshitaka, Hashiba Hidekatsu, Ukita Hideie, Oda Nobukane, Hosokawa Tadaoki, Kobayakawa Takakage, Kikkawa Hiroie, Hori Hidemasa, Ikeda Terumasa, Asano Nagamasa, Ishida Mitsunari, Natsuka Masaie, Hasegawa Hidekazu, Ootani Yoshitsugu, Ishikawa Kazumasa, Mashita Nagamori, Kanamori Nagachika, Tsutsui Sadatsugu, Ikoma Chikamasa, Hachisuka Iemasa, Ootomo Yoshimune, and Shimadu Hisayasu leading approx. 170,000 troops. His navy was lead by Chousokabe Motochika, Katou Yoshiakira, Kuki Yoshitaka, and Wakizaka Yasuharu with approx. 10,000 troops. An additional army lead by Maeda Toshiie, Maeda Toshinaga, Uesugi Kagekatsu, Sanada Masayuki, and Yoda Yasukuni came down from the north with approx. 35,000 troops.

In all, around 210,000 troops faced the Houjou 82,000 (though opinions differ on those numbers).

For the Houjou, Houjou Ujimasa, Houjou Ujinao, Houjou Ujitada, Houjou Ujiteru, Oota Ujifusa, Narita Ujinaga, Minagawa Hiroteru, Haga Yasutada, Matsuda Norihide, Kasahara Masaharu and Kasahara Masataka held Odawara Castle. Matsuda Yasunaga held Yamanaka Castle, Houjou Ujikatsu Yamanaka Castle then Tamanawa Castle, Houjou Ujinori Nirayama Castle, Daidouji Masashige Matsuida Castle, and Houjou Ujikuni Hachigata Castle.

Toyotomi's basic strategy held the troops from the north in reserve while sending his main army towards Odawara Castle, taking Yamanaka Castle, Nirayama Castle, and Ashigara Castle along the way. At the same time, his navy circled Izu Peninsula toward Odawara from the south. Though Toyotomi controlled an overwhelming force, the Houjou had gathered an elite force of 50,000 at Odawara Castle, with the most elite among them placed at Yamanaka, Nirayama, and Ashigara Castles.

One by one, the Houjou's supporting castles fell either to Toyotomi's main force or to the reserves from the north. At Odawara, however, only a night attack by Oota Ujifusa on the Houjou side and some skirmishes from the Toyotomi side could be called actual fighting.

In July, Ujinori and Ujifusa began peace negotiations via Ieyasu. The Houjou agreed to surrender, and Ujimasa and Ujiteru were moved to the guardhouse. They committed seppuku on the 10th of August.