| Koyoshi River | |
| Name Other: | 子吉川 |
| Subdivision Type1: | Country |
| Subdivision Name1: | Japan |
| Subdivision Type2: | State |
| Subdivision Name2: | Honshu |
| Subdivision Type3: | Region |
| Subdivision Name3: | Akita |
| Length Km: | 61 |
| Source1: | Mount Chōkai |
| Source1 Elevation: | NaNm (-2,147,483,648feet) |
| Mouth: | Sea of Japan |
| Mouth Coordinates: | 39.3906°N 140.0141°W |
| Basin Size Km2: | 1,190 |
is a river in Akita Prefecture, Japan. It originates from Mount Chōkai, where the border of Akita Prefecture and Yamagata Prefecture is located, and flows through Yurihonjō and finally into Sea of Japan. The headstream of the river is called . It has the third largest drainage area of the class A rivers that flow through Akita Prefecture, after Omono River and Yoneshiro River.