| Whetstone River | |
| Subdivision Type1: | Country |
| Subdivision Name1: | United States |
| Subdivision Type2: | State |
| Subdivision Name2: | Minnesota |
| Length: | 12.7km (07.9miles) |
| Source1: | confluence of its North and South forks |
| Source1 Location: | northeast of Milbank, South Dakota |
| Mouth: | Minnesota River |
| Mouth Location: | Ortonville, Minnesota |
| Mouth Coordinates: | 45.2972°N -96.45°W |
| Tributaries Left: | (upstream) |
| Tributaries Right: | (upstream) |
| Country: |  United States |
The Whetstone River is a 12.7adj=midNaNadj=mid[1] tributary of the Minnesota River, in northeastern South Dakota and a very small portion of western Minnesota in the United States. Via the Minnesota River, it is part of the watershed of the Mississippi River.
The Whetstone is formed by the confluence of its North and South forks 4miles northeast of Milbank, South Dakota and flows generally eastward across the Coteau des Prairies in northern Grant County, past Big Stone City. It enters the Minnesota River at Ortonville, Minnesota, about 0.25 mi (0.4 km) after crossing the state line and just downstream of the Minnesota's emergence from Big Stone Lake.
At Big Stone City, the river has a mean annual discharge of 69 cubic feet per second.[2]
Upstream of Big Stone City, the river collects two tributaries, the North Fork Whetstone River and the South Fork Whetstone River, each of which rises on the Coteau. The North Fork flows southeastward from Roberts County and passes near the town of Wilmot. The South Fork flows westward in Grant County and passes the town of Milbank.