相模川
Located in the headwaters of the Sagami River, the area around present-day Ōtsuki was heavily settled in the Jōmon period, and over 80 Jōmon sites have been found within city limits. However, there are fewer Yayoi period sites. During the Nara period "ritsuryo" organization of Kai Province, the area came under Tsuru County. From the middle of the Kamakura period, much of the province came under the control of the Takeda clan.