The 1580 battle of Omosu(重須の戦い,Omosu no Tatakai?) was one of many battles fought between the Hōjō and Takeda clans during Japan's Sengoku period. It is distinguished, however, as one of the very few naval battles to be fought in pre-modern Japan.
The battle took place off the coast of the Izu peninsula between the navies of Hōjō Ujimasa, the head of the Hōjō, and those of Takeda Katsuyori, the head of the Takeda. While the navies faced off, the land armies of each family advanced towards one another. In the end, the Hōjō were victorious.