Almost the entire county lies in the Cheyenne River Indian Reservation.

Smaller drainages move runoff water northward from the central-eastern portions to the Missouri River, discharging near the community of Promise.

A significant arm of the Missouri River forms the county's southeastern border.

The county terrain consists of rolling hills, sloping southeastward and dropping off into the Missouri River basin.

Dewey has since the 1990s been a strongly Democratic county in solidly Republican South Dakota.

The last Republican to carry the county was Ronald Reagan in his 1984 landslide when he came within 3,761 votes of claiming all fifty states.

Before this period, by contrast, Dewey was a Republican-leaning county even for South Dakota.