According to the U.S. Census Bureau, the county has a total area of , of which are land and (0.4%) is water. It is the second-largest county in Oklahoma by area. The county lies in the High Plains of the Great Plains physiographic region. It is generally flat, but has some rolling hills. It is drained by the North Canadian River, often called the Beaver River in this area. Tributaries of the river are Coldwater, Hackberry, Goff, Teepee, and Pony Creeks.