Dewey is an unincorporated community near the southwest corner of Custer County, South Dakota, United States, less than one mile from the Wyoming border in a prairie region of the Black Hills.

[6] Dewey lies just to the southwest of the Elk Mountains, a small range that is part of the Black Hills.

The name was changed in 1890 by either the Burlington Railroad which came through the area that year, or the Post Office Department.

[10] Two ranches near Dewey are listed on the National Register of Historic Places in Custer County.

The Tennessee Valley Authority and private uranium companies have explored the Dewey / Burdock area since the 1970s.